Download to read offline
![Now for the part that you've been eagerly waiting for!
In order to execute the program, go to the File menu, and click on Exit. You will get a dialog box asking you
whether you want to save the file or not, say yes. Now that we have typed and saved our file we need to execute
it. The compiler creates executable code. The command used to call the C# compiler is csc followed by the
program name. Since our program name is a.cs, the command csc a.cs will call the compiler. A compiler is a
program which understands the C# programming language. Thus the word class is part and parcel of the C#
language. Microsoft lets you freely download the C# compiler from their web site :
http://msdn.microsoft.com/library/default.asp. Select .Net Framework SDK under .NET Development. Choose
the Download option to download the sdk which is around 127 MB large. Install the product on your machine
cause if you don’t, none of the following programs will work. Also, Internet Explorer 5.5 and Microsoft Data
Access Components(2.7) must be installed prior to installing the sdk
Once done, type the command as follows:
C:csharp>csc a.cs
You will see the following output on your screen in the dos box.
Microsoft (R) Visual C# Compiler Version 7.00.9254 [CLR version v1.0.2914] Copyright (C) Microsoft
Corp 2000-2001. All rights reserved.
error CS5001: Program ‘a.exe’ does not have an entry point defined
Just as the excitement was beginning to grow, our program returns with an error message. Don't worry,
occasional failure is the price of improvement.
The error message starts with an error number CS5001 followed by a cryptic message, which we do not
understand.
We are aware of the fact that everything has a beginning and an end. Similarly, a C# program also has a start
and an end. Ah! Now you realize why the error occurred. We forgot to tell C# where to start executing our
program from. This starting point is also called an entry point.
You can specify the entry point by adding static void Main() to your program, just as we have done below.
a.cs
class zzz
{
static void Main()
{
}
}
Compile the above code giving the command as csc a.cs. Voila! Now no errors.
The compiler will now generate an exe file called a.exe. Giving the command dir at the command prompt will
prove the same. On keen observation you will notice that among the 2 files listed, there is a file by the name
a.exe. Simply type 'a' at the command prompt, your program will now be executed!
C:csharp>a
The program will run but shows no output on our screen. But, at least we get no errors.
The words, static and void, will be explained to you a little later, in the right perspective. Thus if you had felt
the beginnings of a massive headache, you can breathe easy! Anything followed by '(' and ')' brackets is called a
function. So, it is obvious that Main is nothing but a function. Here we are creating a function called Main. It is
followed by the '{' and '}' curly braces. Note that the letter M in Main is capital. Thus C# requires a function
called Main, which is the first function that will be executed. Failure to do so will result in an error. Ergo,](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-7-2048.jpg)
![Arrays
All programming languages embrace the concept of arrays. An array is nothing but more of one entity i.e., a
multiple of the same type. Simply put, when we have five books, we don't just say we have five books. Instead
we say we have an array of five books. So, whenever you want to store multiple values of variables you store
them in an array.
a.cs
class zzz
{
public static void Main()
{
int[] a;
a= new int[3];
a[0]= 1; a[1]= 10; a[2]= 20;
System.Console.WriteLine(a[0]);
a[0]++;
System.Console.WriteLine(a[0]);
int i;
i=1;
System.Console.WriteLine(a[i]);
i=2;
System.Console.WriteLine(a[i]);
}
}
Output
1
2
10
20
Here 'a' is an array of ints. You declare arrays with a set of [] brackets. At this point it is not known how large
the array will be. For that we have a=new int[3]; after new we have int meaning we want to create a variable of
type int. We are putting 3 in the square brackets meaning we want to store 3 ints. This will create three ints a[0],
a[1] and a[2]. They are then initialized to the values 1,10 and 20 respectively. To initialize an array variable we
use the name, i.e. in this case a and follow it with the open and close [] brackets. Inside them we put the array
number. The first variable is called a[0] , the second a[1] and so on. C# like most computer programming
languages likes to start counting from 0 and not 1. Therefore, the last variable is a[2] and not a[3].
Since an array is many of the same type, they all have the same name. In our earlier example, even if we have
many books, all the books will be called books. However, to refer to them individually you can say book1,
book2, book3 etc.
WriteLine will display the value stored in a[0] which is 1. a[0]++ will increment the value stored in a[0] by one.
WriteLine will now display 2.
Thereafter, the variable i is declared as an int. It is then initialized to 1. Within the WriteLine function we have
a[i]. It is not specifically stated which variable, instead we have said a[i]. There is no variable called a[i], but i
has a value 1, so it is read as a[1]. Hence the value stored at a[1], which is 10, is displayed. The next WriteLine
will display the value stored in a[2] as i is reinitialized to 2.
Doing this makes our program more generic. We haven't specifically stated which variable, we are letting a
variable called i decide the name of the variable.](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-48-2048.jpg)
![The next example demonstrates how arrays can be used within loops.
a.cs
class zzz
{
public static void Main()
{
int[] a;
a= new int[3];
int i;
for( i=0; i<=2; i++)
a[i]= i*10;
for( i=0; i<=2; i++)
System.Console.WriteLine(a[i]);
}
}
Output
0
10
20
The advantage of using arrays is that you can decide the name of the variable later and they can also be used in
loops.
Here 'a' is an array of ints and i is a variable of type int. The array size is 3 i.e. it can store three ints. The first for
loop is used to initialize the individual array items. Within the for loop, i is initialized to 0. The condition i<=2
indicates that the loop will execute thrice. So, when the control enters the for loop, for the first time i is 0.
Looking at the right hand side of the expression, i*10 will now read as 0*10, which is 0. Hence, a[i] which is
read as a[0] will be initialized to 0. i++ increments the value of i to 1. The second time a[i] will be read as a[1]
which will be initialized to 10. Thereafter, a[i] will be read as a[2] and initialized to 20 as i is now 2. Now i will
be 3, so the condition evaluates to false and the loop terminates.
The second for loop displays the values stored in the array. This is similar to the above loop. Here we are
displaying the values of the individual array items. As the for loop executes, WriteLine will read a[i] as a[0],
a[1] and a[2] in each case. As such, WriteLine displays 0,10,20. So, starting from the beginning and going upto
the end, all the values stored in the array are displayed.
Arrays can also be used in a foreach statement. This is exemplified in the following program.
a.cs
class zzz
{
public static void Main()
{
int[] a;
a= new int[3];
a[0]= 1; a[1]= 10; a[2]= 20;
foreach ( int i in a)
System.Console.WriteLine(i);
}
}
Output
1
10
20](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-49-2048.jpg)
![The foreach statement lists the elements of the array. It executes a statement for each element of the array or
collection. 'a' is an array of type int that can store three items. a[0], a[1] and a[2] are the elements or items of the
array. They have been initialized to 1, 10 and 20 respectively. In the foreach statement we have ( int i in a ).
i is a variable of type int and a is the array created earlier. The first element of the array 'a' is a[0] and it holds
the value 1. The foreach statement picks up this value and stores it in i. Since i now holds the value 1, WriteLine
displays 1. The second element is a[1]. It picks up its value, which is 10 and stores it in i. Thus i now holds the
value 10. WriteLine will now display 10. This goes on for all the elements of the array. Our array 'a' comprises
three elements. Hence the foreach will execute the WriteLine statement for each of them and display their
values 1,10 and 20. The variable i is only available in the foreach. The foreach makes it easier for us to run
through all the members of any array.
Parameters Revisited with Out and Ref
Variables are the cornerstone of programming languages. In a way, they are like items contained in a bag. You
can put an item in a bag, remove it therefrom, or replace it with another. Our next few programs shall be
explained along these lines.
a.cs
class zzz
{
public static void Main()
{
int i=100;
yyy a;
a=new yyy();
a.abc(i);
System.Console.WriteLine(i);
}
}
class yyy
{
public void abc(int i)
{
System.Console.WriteLine(i);
i=10;
}
}
Output
100
100
'a' is an object that looks yyy. The variable 'i' is initialized to a value 100. Then the object 'a' that looks like yyy
is created. We are now calling the function abc with one parameter 'i'. Within the abc function we are printing
the value of i. When we call the function abc from Main, i had a value 100. So the variable i in abc also has a
value 100. Thus WriteLine will display the value 100. We are now assigning i a new value 10. But the i that
changes is the parameter i in abc, and not the i of Main in zzz. It's like having two similar bags, where you
change the contents of only one. So the WriteLine in Main will again print 100.
Thus the variable i within the abc function is temporary. It exists only within the '{' and '}' braces of the
function. Therefore, this 'i' is local to the function. Its value is not available in Main. It's like your memory,
temporary. It exists as long as you are reading this book. Once the book is over everything is forgotten! Thus
please do not call the variable i in abc as i because it will confuse everyone. Thus i in abc has nothing to do with
the i in Main. Ergo, changing i in abc will have no effect on the i in Main.
But a situation could arise wherein you would like the function abc to change the value of i. In this context you
are saying, I am giving a variable to a function abc, use it, but I would also like the function abc to change it.](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-50-2048.jpg)
![The web server also needs at least one header Content-Type which tells it the type of file being sent across.
Normally a.exe would read the environmental variable QUERY_STRING, extract the parameters, then read a
database, extract some values from it and finally create an HTML file which it would hand over to the web
server. All in all, this is how the magic of the internet is bought about.
Now, let's learn some more C#.
a.cs
class zzz
{
public static void Main(){
string s ;
string [] e;
char[] c ;
s = "hi=bye&no=bad" ;
c = new char[2];
c[0] = '&';
c[1] = '=';
e = s.Split(c);
foreach ( string t in e)
System.Console.WriteLine(t);
}
}
Output
hi
bye
no
bad
s is a string which has been initialized to hi=bye&no=bad which looks similar to what QUERY_STRING as
explained earlier looks like. C is an array of chars which has two members initialized to & and = respectively.
The string class has a member Split, which will split a string as per the delimiters provided as the first
parameters in an array of chars. Thus the split function will take the string s and split it whenever it sees a & or a
= . In this case it will result in 4 strings. Split returns an array of strings which we are storing in an array e and
using foreach to display the array.
Now lets us create a simple form based web application which will accept a table name and then display the
entire table for us.
The html file a.html as usual will be copied to the htdocs sub-directory.
a.html
<html>
<form action= http://127.0.0.1/cgi-bin/a.exe >
<b> Enter Table Name </b>
<input type=text name=aa> <p>
<input type=submit value="Show Table">
<form>
</html>
We will run as:
http://127.0.0.1/a.html
Our C# program freshly compiled and copied to the cgi-bin subdirectory would now read as:
a.cs
using System.Data.OleDb;
class zzz
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-81-2048.jpg)
![public static void Main()
{
try
{
string t;
t=System.Environment.GetEnvironmentVariable("QUERY_STRING");
string [] u;
char [] v = new char[1];
v[0] = '=';
u = t.Split(v);
string w;
w = "select * from " + u[1];
OleDbConnection s;
s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;
Data Source=c:zzz.mdb");
s.Open();
OleDbCommand c;
c=new OleDbCommand(w,s);
OleDbDataReader r;
r = c.ExecuteReader();
System.Console.WriteLine("Content-Type:text/htmln");
while ( r.Read() )
{
for ( int i = 0 ; i < r.FieldCount ; i++)
System.Console.Write(r.GetValue(i)+" ");
System.Console.WriteLine("<br>");
}
}
catch(System.Exception e)
{
System.Console.WriteLine(e.ToString());
}
}
}
At our web form we are asked to write the name of the table. Then click on the button Show Table. The web
browser now writes the following URL in the address bar after we wrote abc as the name of our table.
http://127.0.0.1/cgi-bin/a.exe?aa=abc. The Apache Web Server now runs a.exe. a.exe is nearly similar to what
we wrote earlier with some minor modifications. t contains the value of the environmental variable
QUERY_STRING . v is an array of chars with one member which is our delimiter '='. Split will return an array
of strings in u , in our case aa and abc which will be stored in u[0] and u[1]. Earlier we hard-coded the SQL
Select statement. Here w starts with the constant string 'Select * from ' and the name of the table is got from u[1]
which in turn gets its value from the environmental variable QUERY_STRING. Write the name of any table
that exists in the database in the text box and see how you have been able to web enable your data on the net.
Lets use the table tags to make our data look more appealing in the browser.
a.cs
using System.Data.OleDb;
class zzz
{
public static void Main()
{
try
{
string t;
t=System.Environment.GetEnvironmentVariable("QUERY_STRING");
string [] u;
char [] v = new char[1];
v[0] = '=';
u = t.Split(v);](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-82-2048.jpg)
![string w;
w = "select * from " + u[1];
OleDbConnection s;
s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;
Data Source=c:zzz.mdb");
s.Open();
OleDbCommand c;
c=new OleDbCommand(w,s);
OleDbDataReader r;
r = c.ExecuteReader();
System.Console.WriteLine("Content-Type:text/htmln");
System.Console.WriteLine("<table border=10>");
while ( r.Read() )
{
System.Console.WriteLine("<tr>");
for ( int i = 0 ; i < r.FieldCount ; i++) {
System.Console.Write("<td>" + r.GetValue(i) + "</td>");
}
System.Console.WriteLine("</tr>");
}
System.Console.WriteLine("</table>");
}
catch(System.Exception e)
{
System.Console.WriteLine(e.ToString());
}
}
}
The output looks great. We have made very small additions to our program, mostly cosmetic. We start with the
table tag with a border. The while gets called once for every record and in this while we start and end with a row
tag tr and /tr. The for gets called for every field and here we enclose the field value with a tag td and /td. At the
end of the while we end with the /table tag. The only problem is that we have not displayed the field names at
all. Let the next program do so.
a.cs
using System.Data.OleDb;
class zzz
{
public static void Main()
{
try
{
string t;
t=System.Environment.GetEnvironmentVariable("QUERY_STRING");
string [] u;
char [] v = new char[1];
v[0] = '=';
u = t.Split(v);
string w;
w = "select * from " + u[1];
OleDbConnection s;
s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:zzz.mdb");
s.Open();
OleDbCommand c;
c=new OleDbCommand(w,s);
OleDbDataReader r;
r = c.ExecuteReader();
System.Console.WriteLine("Content-Type:text/htmln");
System.Console.WriteLine("<table border=10>");
System.Console.WriteLine("<tr>");
for ( int i = 0 ; i < r.FieldCount ; i++)](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-83-2048.jpg)
![Lets us know write an insert statement which will ask the user to key in his first and last name and then add it
into a table. We create a table 'bbb' in our database with two fields f1 and f2 both character type. Our html file
also now changes to.
a.html
<html>
<form action= http://127.0.0.1/cgi-bin/a.exe >
<b> Enter Users First Name </b>
<input type=text name=aa> <p>
<b> Enter Users Last Name </b>
<input type=text name=bb> <p>
<input type=submit value="Add">
<form>
</html>
a.cs
using System.Data.OleDb;
class zzz
{
public static void Main()
{
try
{
OleDbConnection s;
s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;
Data Source=c:zzz.mdb");
s.Open();
OleDbCommand c;
string t;
t=System.Environment.GetEnvironmentVariable("QUERY_STRING");
string [] u;
char [] v = new char[2];
v[0] = '=';
v[1] = '&';
u = t.Split(v);
string f1,f2,f3;
f1 = u[1].Replace('+',' ');
f2 = u[3].Replace('+',' ');
f3 = "insert into bbb values('" + f1 + "','" + f2 + "')";
System.Console.WriteLine("Content-Type:text/htmln");
System.Console.WriteLine(f3);
c=new OleDbCommand(f3,s);
c.ExecuteNonQuery();
}
catch(System.Exception e)
{
System.Console.WriteLine(e.ToString());
}
}
}
Output
insert into bbb values('vijay ram','mukhi')
URL in Address Bar.
http://127.0.0.1/cgi-bin/a.exe?aa=vijay+ram&bb=mukhi
The first change is that our array of chars is now two members large and the second delimiter is the '&'. Thus
our QUERY_STRING will break into 4 strings aa , vijay+ram , bb and mukhi in u[0] to u[3]. We replace the +
with the ' ' in both strings and dynamically generate the insert statement. It looks confusing but is not, Once
again f1 and f2 contain the dynamic data but because they are strings we need them to be separated by a single](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-85-2048.jpg)
![inverted comma. Thus the complication in concatenating strings. The browser displays the SQL statement that
will be executed, but in real life we would display a success or error message.
Data Search Program
Now let us write a search engine or more precisely, a general purpose data retrieval engine. For which we have
to first create a simple database that will contain the following fields :
Keyword - that will store what the page contains,
url - the address of the page,
name - to be displayed instead of the url
and finally the text that will be displayed explaining the page.
create table ss ( keyword char(10), url char(100), name char(100), descrip char(100));
We will also add the following records.
insert into ss values ( 'java', 'a.html', 'java site', 'great java site');
insert into ss values ( 'c', 'b.html', 'c site', 'great c site');
insert into ss values ( 'basic', 'c.html', 'basic site', 'great basic site');
insert into ss values ( 'cobol', 'd.html', 'cobol site', 'great cobol site');
Create the following html files in the htdocs subdirectory
search.html
<html>
<form action= http://127.0.0.1/cgi-bin/a.exe >
<b> Enter keyword </b>
<input type=text name=aa> <p>
<input type=submit value="Search">
<form>
</html>
a.html
The file is for java
b.html
The file is for C
c.html
The file is for Basic
d.html
The file is for Cobol
The C# program will be as follows
a.cs
using System.Data.OleDb;
class zzz
{
public static void Main()
{
try
{
string t;
t=System.Environment.GetEnvironmentVariable("QUERY_STRING");
string [] u;
char [] v = new char[1];
v[0] = '=';
u = t.Split(v);
string w;](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-86-2048.jpg)
![System.Console.WriteLine("Content-Type:text/htmln");
w = "select * from ss where keyword='" + u[1] + "'";
OleDbConnection s;
s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:zzz.mdb");
s.Open();
OleDbCommand c;
c=new OleDbCommand(w,s);
OleDbDataReader r;
r = c.ExecuteReader();
while ( r.Read() )
{
System.Console.WriteLine("<a href=/{0}> {1} </a> {2}
<br>",r.GetValue(1),r.GetValue(2),r.GetValue(3));
}
}
catch(System.Exception e)
{
System.Console.WriteLine(e.ToString());
}
}
}
Open up the browser and enter the url 'http://127.0.0.1/search.html'
All that this program does is use the concept of a hyperlink to build a search engine and extract data from a
database. We assume we have a database of a trillion records that map the internet telling us the keywords that
each url stands for. The user keys in the name of a language and we use the select statement to fetch the records
meeting the condition, format it and then send the html file across.
We can use the same concept to fill up a list box. The html tags for list box are as follows
<select name = aa>
<option> blue </option>
<option> red</option>
<option> orange </option>
</select>
In a real life situation, the colours will be retrieved from a database and the HTML file generated by a
CGI program.
6
Miscellaneous
Multiple files
A C# program can be spread over multiple files. So far all our code has been written in one large file. Lets us
create 2 .cs files a.cs and b.cs as follows
a.cs
class zzz
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-87-2048.jpg)
![Compiler Error
Microsoft (R) Visual C# Compiler Version 7.00.9254 [CLR version v1.0.2914]
Copyright (C) Microsoft Corp 2000-2001. All rights reserved.
a.cs(10,6): error CS0663: 'abc' cannot define overloaded methods which differ only on ref and out
a.cs(6,6): (Location of symbol related to previous error)
The signature consists of not only the parameter data types, but also the kind of parameters i.e. out ref etc. As
function abc takes an int with different modifiers i.e. out etc, the signature on each is different. The signature of
a method consists of its name and number and types of its formal parameters. The return type of a function is
not part of the signature. No two functions can have the same signature and also non-members cannot have the
same name as members.
A function/method can be called by four different types of parameters. These are pass by value, reference,
output and finally parameter arrays. The parameter modifier is not part of the function signature. Lets now
understand what parameter arrays are all about.
Params Parameter
A method declaration creates a separate declaration space. This means that anything created in a method is lost
at the end of the method.
a.cs
public class zzz
{
public void abc(int i,string i) {}
public void pqr(int i)
{
string i;
}
public static void Main()
{
}
}
Compiler Error
a.cs(3,23): error CS0100: The parameter name 'i' is a duplicate
a.cs(6,8): error CS0136: A local variable named 'i' cannot be declared in this scope because it would
give a different meaning to 'i', which is already used in a 'parent or current' scope to denote something
else
Parameter names have to be unique. Also, we cannot have a parameter and a variable created in a function block
with the same name.
In pass by value, the value of the variable is passed. In the case of ref and out, the address of the reference is
passed.
a.cs
public class zzz
{
string s = "hi";
public static void Main()
{
zzz z = new zzz();
z.pqr();
}
void pqr()
{
abc(ref s,ref s);
System.Console.WriteLine(s);](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-99-2048.jpg)
![}
void abc(ref string a, ref string b) {
System.Console.WriteLine(s);
a="no";
System.Console.WriteLine(s);
b = "yes";
System.Console.WriteLine(s);
s = "maybe";
}
}
Output
hi
no
yes
maybe
You are allowed to pass the same ref parameter as many times as you desire. In the function abc the string s has
a value of hi. Then by changing the string b to no, we are actually changing the string s to no as s is passed by
reference. Variables a and s refer to the same string in memory. Changing one changes the other. Again
changing b also changes s as they refer to the same string. Thus variables a, b and s refer to the same string in
memory.
a.cs
public class zzz
{
public static void Main()
{
zzz z = new zzz();
z.pqr();
}
void pqr()
{
abc(2,"hi","bye","no");
abc(20,"hi");
abc(2);
}
void abc(int i , params string [] b)
{
foreach ( string s in b)
System.Console.WriteLine(s + " " + i);
}
}
Output
hi 2
bye 2
no 2
hi 20
We will encounter a situation where we would like to pass a variable number of arguments to a function. This is
not possible as of now as C# is extremely finicky about the number and type of data we pass to a function. If we
pass a string where an int is expected, C# starts screaming like a baby. If we want to pass a variable number of
arguments to a function, we have to use a keyword params. This keyword can only be applied to the last
parameter. Therefore the variable number of arguments can only come at the end. In the case of function abc,
the first parameter has to be an int, the rest of them can be from zero to an infinite number of strings.
a.cs
public class zzz
{
public static void Main()
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-100-2048.jpg)
![}
void abc(int i , params string [] b , int j)
{
}
}
Compiler Error
a.cs(6,18): error CS0231: A params or __arglist parameter must be the last parameter in a formal
parameter list
The params keyword in this version has to be at the end only as stated above.
a.cs
public class zzz
{
public static void Main()
{
zzz z = new zzz();
z.pqr();
}
void pqr()
{
abc(2,3,4);
abc(20,1);
abc(2);
}
void abc(int i , params int [] b)
{
foreach ( int s in b)
System.Console.WriteLine(s + " " + i); } }
Output
3 2
4 2
1 20
C# is smart enough if the penultimate parameter and the params have the same data type. The first int is stored
in the variable i, the rest are made part of the array b.
a.cs
public class zzz
{
void abc(int i , params string [][] b)
{
}
void abc(int i , params string [,] b)
{
}
}
Compiler Error
a.cs(6,6): error CS0225: The params parameter must be a single dimensional array
The data type of the params parameter must be, as the error message states, a single dimensional array. Thus [][]
is allowed but not [,]. You are also not allowed to combine the params keyword with ref or out.
a.cs
public class zzz
{
public static void Main()
{
zzz z = new zzz();
z.pqr();](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-101-2048.jpg)
![}
void pqr()
{
string [] s = {"hi","bye","no"};
abc(2,s);
}
void abc(int i , params string [] b)
{
foreach ( string s in b)
System.Console.WriteLine(s + " " + i);
}
}
Output
hi 2
bye 2
no 2
You are allowed to pass an array of strings instead of individual strings as parameters. Here s is an array of
strings which has been initialized using the short form. Internally when we call the function abc, C# converts the
array of strings into individual strings.
a.cs
public class zzz
{
public static void Main()
{
zzz z = new zzz();
z.pqr();
}
void pqr()
{
string [] s1 = {"hi","bye"};
abc(2,s1,"hell");
}
void abc(int i , params string [] b)
{
}
}
Compiler Error
a.cs(11,1): error CS1502: The best overloaded method match for 'zzz.abc(int, params string[])' has
some invalid arguments
a.cs(11,7): error CS1503: Argument '2': cannot convert from 'string[]' to 'string'
Mixing and matching is not allowed in C#. What we assumed C# would do is to add the last string hell to the
array of strings s1 or convert s1 to individual strings and then add the string hell to it. Perfectly logical we
thought. Only if wishes were horses…
Internally before calling the function abc, it collects all the individual parameters and converts it into one big
array for the params statement.
a.cs
public class zzz
{
public static void Main()
{
zzz z = new zzz();
z.pqr();
}
void pqr()
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-102-2048.jpg)
![int [] a = {1,2,3};
abc(2,a);
System.Console.WriteLine(a[1]);
}
void abc(int i , params int [] b)
{
b[1] = 100;
}
}
Output
100
The output produced is proof of concept. The array member a[1] has an initial value of 2 and in the function abc
we change it to 100. The original changes, this means that the array is given to the function abc.
a.cs
public class zzz
{
public static void Main()
{
zzz z = new zzz();
z.pqr();
}
void pqr()
{
int a = 10;
abc(2,100,a,20);
System.Console.WriteLine(a);
}
void abc(int i , params int [] b)
{
b[1] = 100;
}
}
Output
10
In this case C# creates an array containing 100 10 and 20. We are changing the second member to 100 which
has nothing to do with the variable a. As abc has no knowledge of a, how on earth can abc change the value of
the int a? Thus it stays the same.
a.cs
public class zzz
{
public static void Main()
{
zzz z = new zzz();
z.pqr();
}
void pqr()
{
abc(2);
abc(2,3);
abc(2,3,5,6);
}
void abc(int i, int j)
{
System.Console.WriteLine("two ints "+ i + " " + j);
}
void abc(params int [] a)
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-103-2048.jpg)
![System.Console.WriteLine("params a");
}
}
Output
params a
two ints 2 3
params a
Here we are discussing function overloading. C# is extremely bright though partial. It does not like the params
statement and treats it like a stepchild. When we call abc with one int, C# can only call the abc that takes a
params as a parameter as it matches one int. An array can contain one member. The fun starts with the abc that
is being called with two ints. Here we have a dilemma. C# can call the params abc or the abc with two ints. As
mentioned earlier, C# treats the params as a second class citizen and therefore chooses the abc with two ints.
When there are more than two ints like in the third invocation, C# has no choice but to grudgingly choose the
abc with the params. C# chooses the params as a last resort before flagging an error.
a.cs
using System;
class zzz
{
static void ff(params object[] b) {
foreach (object o in b) {
Console.Write(o.GetType().FullName + " ");
}
Console.WriteLine();
}
static void Main() {
object[] a = {1, "Hello", 123.456};
object o = a;
ff(a);
ff((object)a);
ff(o);
ff((object[])o);
}
}
Output
System.Int32 System.String System.Double
System.Object[]
System.Object[]
System.Int32 System.String System.Double
In the first case we are passing the function ff an array of object that looks like object. We will tell you a little
later that all classes are derived from object. The function ff receives an array of objects b. In the foreach we
know that the object class has a function GetType that returns an object that looks like Type, which in turn has a
function called FullName which returns the name of the type. We see three different types displayed. In the
second invocation of ff we are casting a to an object. There is no conversion available from converting an object
to an object array i.e. object []. Therefore a one element object [] is created. It's the same case in the third
invocation and the last explicitly casts to an object array.
a.cs
using System;
class zzz
{
static void ff(params object[] b) {
Console.WriteLine(b.GetType().FullName);
Console.WriteLine(b.Length);
Console.WriteLine(b[0]);
}
static void Main() {
object[] a = {1, "Hello", 123.456};](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-104-2048.jpg)
![ff((object)a);
}
}
Output
System.Object[]
1
System.Object[]
Inheritance
a.cs
class zzz
{
public static void Main()
{
xxx a = new xxx();
a.abc();
}
}
class yyy
{
public int i = 10;
public void abc()
{
System.Console.WriteLine("yyy abc");
}
public void pqr()
{
System.Console.WriteLine("yyy pqr");
}
}
class xxx
{
}
Compiler Error
a.cs(6,1): error CS0117: 'xxx' does not contain a definition for 'abc'
The class yyy contains 2 functions and one instance variable. The class xxx contains no code and no variables at
all. An empty class does not denote any error as we are able to instantiate an object that looks like xxx. The
error comes about because the class xxx has no function called abc. However the class yyy has a function abc.
Would it not be great if we were allowed to use all the code in the class yyy from xxx. Easier said than done,
we guess!
a.cs
class zzz
{
public static void Main()
{
xxx a = new xxx();
a.abc();
}
}
class yyy
{
public int i = 10;
public void abc()
{
System.Console.WriteLine("yyy abc");
}
public void pqr()
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-105-2048.jpg)
![public class zzz
{
public static void Main()
{
yyy a = new yyy();
a[1] = 10;
}
}
public class yyy {
}
Compiler Error
a.cs(6,1): error CS0021: Cannot apply indexing with [] to an expression of type ‘yyy’
We have created an object a that looks like yyy. The object a, in no sense of the word is an array. We are
assuming that a is an array and we’ve used the array syntax a[], hence it gives us an error.
a.cs
public class zzz
{
public static void Main()
{
yyy a = new yyy();
a[1] = 10;
}
}
public class yyy
{
public int this[int i]
{
set {
System.Console.WriteLine(“in get “ + value + “ “ + i);
}
}
}
Output
in get 10 1
We’ve added a few lines to have the array notation work with an object that looks like yyy. To implement
indexers, we need to create a special property called this. This is a reserved word. As of now, we have a
parameter i (an int) in the square brackets. When we did properties earlier,we learnt that a set gets called
whenever we want to initialize or set a variable. Within the set accessor we have a special variable called value
which stores the value passed to the set, in this case 10. The variable i will hold the value 1 as the array
parameter is 1.
This is how we implement arrays when there are none.
a.cs
public class zzz
{
public static void Main()
{
yyy a = new yyy();
System.Console.WriteLine(a[1]);
}
}
public class yyy
{
public int this[int i]
{
set
{
System.Console.WriteLine(“in get “ + value + “ “ + i);](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-153-2048.jpg)
![}
get
{
System.Console.WriteLine(“in set “ + i);
return 23;
}
}
}
Output
in set 1
23
The rules binding properties are applicabe to indexers too. When you want to read the value of a[1], the get gets
called. The major difference between properties and indexers is that when you implement the code for indexers
you have to understand that the get and set get called with a variable which is the array parameter value. The
code will have to understand array simulation.
a.cs
public class zzz
{
public static void Main()
{
yyy a = new yyy();
a[“hi”] = 30;
System.Console.WriteLine(a[“hi”]);
}
}
public class yyy
{
public int z;
public int this[string i]
{
set
{
System.Console.WriteLine(“in get “ + value + “ “ + i);
z = value;
}
get
{
System.Console.WriteLine(“in set “ + i);
return z;
}
}
}
Output
In get 30 hi
In set hi
30
The this property has a return value, in this case, an int. Also the [] brackets can contain data types other than an
int. In this case a string. The string i has a value hi as that is what we passed in the array brackets. You can have
two this’s in your class. You have to decide what data type to use in the array brackets. An indexer is very
useful when you have a database object and you want to access the data in the fields using a notation
[“fieldname”]
Indexers follow the same concepts of virtual, new, override etc.
a.cs
class zzz
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-154-2048.jpg)
![public static void Main()
{
yyy a = new yyy();
a[1] = 10;
a[“one”] = 10;
a[“hi”,2] = 30;
}
}
class yyy
{
public int this [ int i]
{
set
{
System.Console.WriteLine(“one int “+ i + “ “ + value);
}
}
public int this [ string i]
{
set
{
System.Console.WriteLine(“one string “+ i + “ “ + value);
}
}
public int this [ string i, int j]
{
set
{
System.Console.WriteLine(“one string and int “+ i + “ “ + j + “ “ + value);
}
}
}
Output
one int 1 10
one string one 10
one string and int hi 2 30
The signature of an indexer is the number and types of formal parameters. The return value and the names of the
parameters do not contribute to the indexers signature. Thus we have overloaded the indexers to take an int,
string or a string int combination. Each time a different function gets called. The point to understand is that all
the indexers have to return the same data type, in our case int. The same rules that apply to function overloading
apply here also. Functions cannot differ only by return values. We are sure that for indexers in the next version,
C# should/must make an exception.
A property is identified by its name, an indexer by its signature. There is no concept of property overloading in
C#.
a.cs
class zzz
{
public static void Main()
{
}
}
class yyy
{
public static int this [ int i]
{
set
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-155-2048.jpg)
![}
}
}
Compiler Error
a.cs(9,19): error CS0106: The modifier ‘static’ is not valid for this item
A property can be both an instance member which is the default or static. An indexer unfortunately can only be
an instance member and not static. God alone knows why this discrimination against indexers. Once again no
rational reason for the above error. Obviously you cannot create a variable with the same name as that of the
parameter passed in the indexer.
a.cs
class zzz
{
public static void Main()
{
xxx a = new xxx();
a[2] = 20;
System.Console.WriteLine(a[2]);
}
}
class yyy
{
public virtual int this [ int i]
{
get
{
System.Console.WriteLine(“yyy get “ + i);
return 20;
}
set
{
System.Console.WriteLine(“yyy set “ + value + “ “ + i);
}
}
}
class xxx : yyy
{
public override int this [ int i]
{
get
{
int p = base[i];
System.Console.WriteLine(“xxx get “ + i + “ “ + p);
return 200;
}
set
{
System.Console.WriteLine(“xxx set “ + value + “ “ + i);
base[i] = value;
}
}
}
Output
xxx set 20 2
yyy set 20 2
yyy get 2
xxx get 2 20
200](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-156-2048.jpg)
![The above example deals with calling the indexers of the base class. At times when we are overriding code in
the derived class, we would like to call the original indexer in the base class first. The first rule that we have to
adhere to is that the indexer in the base class must be declared virtual. In the derived class, we are now declaring
it with the modifier override. Same rules as above. In the set accessor, we have to call the original as base[i],
where i is the index to the indexer. Also we need to pass it the value to initialize itself. This is stored in the
variable value. This a[2] in Main gets replaced by base[2] in the set. In get the reverse takes place. Here we need
to place base[i] on the right of the equalto sign, the original get will return a value, in this case 20, which we are
storing in a variable p. What we do with p as well as the value from the get is our business.
a.cs
class yyy
{
public int this [ byte i , string j]
{
get
{
return 10;
}
set
{
}
}
int get_Item(byte i,string j)
{
return 20
}
void set_Item(byte i,string j , int value)
{
}
}
Compiler Error
a.cs(5,1): error CS0111: Class ‘yyy’ already defines a member called ‘get_Item’ with the same
parameter types
a.cs(9,1): error CS0111: Class ‘yyy’ already defines a member called ‘set_Item’ with the same
parameter types
Like a property, an indexer also gets a name change. If people can get their bodies pierced then why
cannot a indexer get converted to a series of functions starting with get? For a get, the parameters are
the same as we pass to an indexer. It has a return value and the type of the indexer. Also the set has
one more added parameter and that is the free variable value.
11
Interfaces and Structures
Constructors Revisited
A constructor is used to initialize the instance of a class as explained earlier.
a.cs
public class zzz](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-157-2048.jpg)
![}
}
public delegate void d1();
public interface aaa
{
void a1(string s);
int a2
{
get; set ;
}
event d1 ddd;
string this[int i]
{
get; set;
} }
The above example demonstrates the four entities an interface can contain. Anything else will flag an error.
a.cs
class zzz
{
public static void Main()
{
ccc c = new ccc();
aaa a = c;
bbb b = c;
c.a1();c.a2();
a.a1();
b.a1();b.a2();
}
}
interface aaa
{
void a1();
}
interface bbb : aaa
{
void a2();
}
class ccc : bbb
{
public void a1()
{
System.Console.WriteLine("a1");
}
public void a2()
{
System.Console.WriteLine("a2");
}
}
Output
a1
a2
a1
a1
a2
An interface can also, like a class, inherit from one or more interfaces. In this case bbb as an interface, can
inherit from aaa. The class ccc inherits from interface bbb and and thus interface aaa has to implement
functions a1 and a2. The object c not only looks like a ccc but also looks like an aaa and bbb. Thus equating
them does not give an error. The reverse, however, is not true. It will obviously flag an error. However a can
only access functions from the interface it belongs to, in this case a1 and b can access a1 and a2. b cannot access](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-175-2048.jpg)
![{
public bool MoveNext()
{
System.Console.WriteLine(“MoveNext”);
return true;
}
public void Reset()
{
System.Console.WriteLine(“Reset”);
}
public object Current
{
get
{
System.Console.WriteLine(“Current”);
return “hi”;
}
}
}
Run the program and you will notice that the output does not stop. It goes on forever.
IEnumerator is an interface which belongs to the namespace System.Collections. foreach first calls the function
GetEnumerator from yyy. It expects this function to return an object like IEnumerator. It then calls the function
MoveNext from this returned object. If MoveNext returns true it knows that there is some data to be read and it
calls the property Current to access this data. From Current the get accessor gets called which always returns
“hi” in our case. Then MoveNext gets called and if it returns false, we quit out of the foreach statement. As
MoveNext always returns true, we go into an indefinite loop.
a.cs
using System.Collections;
public class zzz
{
public static void Main()
{
yyy f = new yyy();
foreach (string i in f)
{
System.Console.WriteLine(i);
}
}
}
class yyy
{
public IEnumerator GetEnumerator()
{
return new xxx();
}
}
class xxx : IEnumerator
{
public string [] a = new string[3] {“hi” , “bye” ,”no”};
public int i = -1;
public bool MoveNext()
{
i++;
System.Console.WriteLine(“MoveNext” + i);
if ( i == 3)
return false;
else
return true;
}
public void Reset()](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-209-2048.jpg)
![{
System.Console.WriteLine(“Reset”);
}
public object Current
{
get
{
System.Console.WriteLine(“Current “ + a[i]);
return a[i];
}
}
}
Output
MoveNext0
Current hi
hi
MoveNext1
Current bye
bye
MoveNext2
Current no
no
MoveNext3
We have created an array a which has 3 members and initialized them respectively to hi, bye and no by giving
the strings in {} immediately after the new. Each time MoveNext gets called the variable i is increased by 1. If
the value of i is 3, we have no more strings to return and thus we return false, else we return true. The variable i
keeps track of how many times the function MoveNext is being called. As MoveNext returns true, Current gets
called which returns a string from the array using i as the offset. Thus we can iterate through the entire array
depending upon the length.
a.cs
using System.Collections;
public class zzz
{
public static void Main()
{
yyy f = new yyy(“This is Great”);
foreach (string i in f)
{
System.Console.WriteLine(i);
}
}
}
class yyy
{
string t;
public yyy(string t1)
{
t = t1;
}
public IEnumerator GetEnumerator()
{
return new xxx(t);
}
}
class xxx : IEnumerator
{
public string [] a;
public xxx(string t3)
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-210-2048.jpg)
![char [] b = new char[1];
b[0] = ‘ ‘;
a = t3.Split(b);
}
public int i = -1;
public bool MoveNext()
{
i++;
System.Console.WriteLine(“MoveNext “ + i);
if ( i == a.Length)
return false;
else
return true;
}
public void Reset()
{
System.Console.WriteLine(“Reset”);
}
public object Current
{
get
{
System.Console.WriteLine(“Current “ + a[i]);
return a[i];
}
}
}
Output
MoveNext 0
Current This
This
MoveNext 1
Current is
is
MoveNext 2
Current Great
Great
MoveNext 3
Pretty big program. At the time of creating a yyy object we are passing a string to the constructor.
Thus the yyy constructor gets called first. The constructor stores this string in variable t. The foreach
statement calls GetEnumerator which now creates a xxx object passing it the string through t. The
constructor of xxx now gets called. Every string class has a member function called Split. Split will
break up a string on certain characters which we call delimiters. In this case, we want our string to be
broken up whenever we encounter a space. The Split function requires an array of chars which it can
use as a delimiter. The reason it requires an array is because we may have more than one char that we
would like to break the string on. Like earlier, the array a now contains the array of strings. The last
change is the condition in the if statement. Earlier we used a constant number, now we use a member,
Length, of an array which stores the length of the array or the number of members. Thus the class yyy
can now be used as a collection class which enumerates the individual words in the string. The
function Reset for some reason never ever gets called.](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-211-2048.jpg)
![14
Attributes, The Reflection API
And Conditionals
Attributes
a.cs
class zzz
{
public static void Main()
{
}
}
[vijay]
class yyy
{
}
Compiler Error
a.cs(7,2): error CS0246: The type or namespace name 'vijay' could not be found (are you missing a
using directive or an assembly reference?)
Anything in a square bracket is called an attribute. We tried to create an attribute called vijay, which C#, for
some reason, does not seem to recognize.
a.cs
class zzz
{
public static void Main()
{
}
}
class vijay : System.Attribute
{
}
[vijay]
class yyy
{
}
All that we have done is created a class vijay that has been derived from the class System.Attribute and the error
simply disappears. Thus an attribute is simply a class that derives from System.Attribute. To understand
attributes lets take an example with structures.
a.cs
class zzz
{
public static void Main()
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-212-2048.jpg)
![yyy a = new yyy();
a.i = 65536+512+3;
System.Console.WriteLine(a.i + " " + a.j + " " + a.k);
}
}
struct yyy {
public int i;
public short j;
public byte k;
}
Output
66051 0 0
A simple revision once again. We have created a structure a, that looks like yyy and initialized only one member
i. Hence we see the warnings. The other members j and k get a default value of zero.
a.cs
using System;
class zzz
{
unsafe public static void Main()
{
Console.WriteLine(sizeof(byte) + " " + sizeof(short) + " " + sizeof(int) + " " + sizeof(long));
}
}
>csc a.cs
Compiler Error
a.cs(4,27): error CS0227: Unsafe code may only appear if compiling with /unsafe
The error here says that you have to use the /unsafe option while compiling any unsafe code.
>csc a.cs /unsafe
Output
1 2 4 8
We shall explain the modifier unsafe in the next chapter. Sizeof tells us how much memory C# allocates for a
data type. A byte is allocated one memory location, short 2, int 4 and a long 8.
a.cs
using System.Runtime.InteropServices;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.i = 65536+512+3;
System.Console.WriteLine(a.i + “ “ + a.j + “ “ + a.k);
}
}
[StructLayout(LayoutKind.Explicit)]
struct yyy
{
[FieldOffset(0)] public int i;
[FieldOffset(0)] public short j;
[FieldOffset(0)] public byte k;
}](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-213-2048.jpg)
![Output
66051 515 3
We are using an attribute StructLayout that belongs to the namespace System.Runtime.InteropServices. In the
earlier program, we had used an attribute called vijay. Thus, StructLayout is a class derived from Attribute. We
are passing a parameter LayoutKind.Explicit to it. The output now differs dramatically.
Every variable is stored in memory. FieldOffset indicates the starting position of the variable within the memory
location. Offset of 0 will position i, j, and k, all three variables at the same memory address of a. Explicit
requires FieldOffset to be mentioned as we are explicitly laying the order for the variables held in the
strucuture.LayoutKind.Sequential and LayoutKind.Auto gives different memory locations to each of the
variable.
We will explain the reasons a little later in the coming chapter ‘Unsafe Code’. We have seen how important
attributes are so lets delve deeper into them.
a.cs
class zzz
{
public static void Main()
{
}
}
class vijayAttribute : System.Attribute
{
}
[vijay]
class yyy
{
}
[vijayAttribute]
class yyy1
{
}
We are allowed a little leeway in the name of the attribute. By convention, the attribute class should end with
the word Attribute and when we use the attribute, the name attribute is optional.
a.cs
class zzz
{
public static void Main()
{
}
}
class vijay : System.Attribute
{
}
[vijay("hi")]
class yyy
{
}
Compiler Error
a.cs(10,2): error CS1501: No overload for method 'vijay' takes '1' arguments
We had used the attribute StructLayout earlier where we passed a parameter. When we do the same thing with
our attribute vijay, we get the above error.
a.cs
class zzz](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-214-2048.jpg)
![{
public static void Main()
{
}
}
class vijay : System.Attribute
{
public vijay(string s)
{
}
}
[vijay("hi")]
class yyy
{
}
We forgot to add a constructor that accepts a string as a parameter. If we had passed a number to our attribute
vijay, we would have to create a constructor that accepts an int. Thus if we pass 2 parameters to vijay, we need
the appropriate constructor.
a.cs
class zzz
{
public static void Main()
{
}
}
class vijay : System.Attribute
{
public vijay(string s ,int i)
{
}
}
[vijay("hi",10,mukhi = 200)]
class yyy
{
}
Compiler Error
a.cs(13,16): error CS0103: The name 'mukhi' does not exist in the class or namespace 'vijay'
What we tried to do is, take a word called mukhi and initialize it to 200. C# comes back and tells us that it does
not know what mukhi is.
a.cs
class zzz
{
public static void Main()
{
}
}
class vijay : System.Attribute
{
public vijay(string s ,int i)
{
}
public int mukhi;
}
[vijay("hi",10,mukhi = 200)]
class yyy
{
}](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-215-2048.jpg)
![mukhi, now, is called a named parameter. It can also be termed as a property.
a.cs
class zzz
{
public static void Main()
{
}
}
class vijay : System.Attribute
{
public vijay(string s ,int i)
{
}
public int mukhi;
public string sonal
{
get
{
return "ss";
}
set
{
;
}
}
}
[vijay("hi",10, mukhi = 200, sonal = "bye")]
class yyy
{
}
A named parameter is a non-static field or a non-readonly property. A positional parameter is what we pass on
to a constructor. We have 2 positional parameters as our constructor has two parameters and mukhi and sonal
are our named parameters. The named parameters come after the positional ones. The positional parameter's
order is important, but the named parameters can be in any order. If we don't follow this rule we will get an
error as follows.
Compiler Error
a.cs(19,22): error CS1016: Named attribute argument expected
When we place the attribute before a function, the error disappears.
a.cs
using System.Runtime.InteropServices;
using System;
class zzz
{
public static void Main()
{
}
}
[AttributeUsage(AttributeTargets.Class)]
class vijay : System.Attribute
{
public vijay(string s ,int i)
{
}
public int mukhi;
public string sonal](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-216-2048.jpg)
![{
get { return "ss"; }
set { ; }
}
}
class yyy
{
[vijay("hi",10, sonal = "bye",mukhi = 200 )]
public void abc() {}
}
Compiler Error
a.cs(24,2): error CS0592: Attribute 'vijay' is not valid on this declaration type. It is valid on 'class'
declarations only.
AttributeUsage is one more attribute class derived from Attribute. It gives us the option to decide where the user
can use the Attribute. The parameter in this case is class and hence we can use it only in front of a class and not
in front of a method. The default is anywhere.
The Reflection API
Reflection or Introspection is when you look within to find out about your true self. In the same way we need a
method by means of which, our program can find out all about a class. We need to know how many methods,
properties etc while our program is executing or running. This distinction is important and we could always read
the documentation if we wanted to know more about the functionality of a class. But, C# gives us a large
number of functions that tell us the innards of a class. These functions put together have to be used in a certain
way. The functions have to be called in a certain order and the parameters to them have to conform to certain
data types. This concept is called an API or a Application Program Interface. In short, an API is how a
programmer uses functions to get a desired result.
a.cs
using System;
class zzz
{
public static void Main()
{
Type m;
m = typeof(int);
System.Console.WriteLine(m.Name + " " + m.FullName);
m = typeof(System.Int32);
System.Console.WriteLine(m.Name + " " + m.FullName);
m = typeof(yyy);
System.Console.WriteLine(m.Name + " " + m.FullName);
}
}
class yyy
{
}
Output
Int32 System.Int32
Int32 System.Int32
yyy yyy
Typeof is a keyword. It needs a class name as a parameter. In the first case, we are specifying a class called int.
typeof returns an object that looks like Type. This class has two members Name, which gives the name of the
class and FullName which is the name preceded with the name of the Namespace. When we use int as the name
of the class, the member Name does not display int but Int32; we mentioned earlier int is an alias for a structure
Int32 in the System namespace. This is what FullName tells us. We thus have visible proof that int is an alias for
a structure. yyy is class belonging to no namespace and hence the Name and FullName members are similar.](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-217-2048.jpg)
![a.cs
using System;
using System.Reflection;
class zzz
{
public static void Main()
{
Type m = typeof(yyy);
MemberInfo [] n;
n = m.GetMembers();
Console.WriteLine(n.Length);
foreach ( MemberInfo a in n)
{
Console.WriteLine(a.Name);
}
}
}
class yyy {
public void abc() {}
private int pqr( int i ) { return 0;}
protected string xyz (string g , int p) {return "";}
}
Output
6
GetHashCode
Equals
ToString
abc
GetType
.ctor
We are now displaying the members of a class yyy. The class Type has a function called GetMembers that
returns an array of type MemberInfo. Every array has a field called Length that returns the size of the array. In
our specific case it is 6. We then use a foreach to run through each member of the MemberInfo array and are
displaying the name of each function using the field Name from the class MemberInfo.
a.cs
using System;
using System.Reflection;
class zzz
{
public static void Main()
{
Type m = typeof(yyy);
MemberInfo [] n;
n = m.GetMembers();
Console.WriteLine(n.Length);
foreach ( MemberInfo a in n)
{
Console.WriteLine((MemberInfo)a + " " + a.DeclaringType);
}
}
}
class yyy {
public int i;
public void abc() {}
public int pqr( int i ) { return 0;}
public string xyz (string g , int p) {return "";}
}
Output
9](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-218-2048.jpg)
![Int32 i yyy
Int32 GetHashCode() System.Object
Boolean Equals(System.Object) System.Object
System.String ToString() System.Object
Void abc() yyy
Int32 pqr(Int32) yyy
System.String xyz(System.String, Int32) yyy
System.Type GetType() System.Object
Void .ctor() yyy
The first concept you need to be clear with is that we can inspect only details of public members and protected
or private like pqr and xyz. Also variables are part of the members of a class. The MemberInfo object has a
ToString function that displays the entire function in all its glory including parameters and their data types. The
names of the parameter variables are however not being displayed. The DeclaringType member returns the class
name that the member belongs to. Thus we can differentiate which class created the function.
Let us now display the attributes used on a class.
a.cs
using System.Runtime.InteropServices;
using System;
using System.Reflection;
class zzz
{
public static void Main()
{
Type m;
m = typeof(yyy);
System.Console.WriteLine(m.Name);
foreach(object a in m.GetCustomAttributes (true))
Console.WriteLine(a);
}
}
[AttributeUsage(AttributeTargets.All)]
class vijay : System.Attribute
{
string s1,s2;int i1;
public int mukhi;
public override string ToString()
{
return s1+" " + s2+" " + i1 + " " + mukhi;
}
public vijay(string s ,int i)
{
s1=s;i1=i;
}
public string sonal
{
get { return s2; }
set { s2 = value; }
}
}
[vijay("hi1",10, sonal = "bye1",mukhi = 200 )]
class yyy
{
[vijay("hi2",100, sonal = "bye2",mukhi = 2000 )]
public void abc() {}
[vijay("hi3",1000, sonal = "bye3",mukhi = 2 )]
public int i;
}
Output](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-219-2048.jpg)
![yyy
hi1 bye1 10 200
GetCustomAttributes takes a boolean as parameter and returns an array of objects. The ToString function of the
attribute class gets called which will decide what string the attribute stands for.
a.cs
using System.Runtime.InteropServices;
using System;
using System.Reflection;
class zzz
{
public static void Main()
{
Type m;
m = typeof(yyy);
System.Console.WriteLine(m.Name);
foreach(MethodInfo a in m.GetMethods())
{
object [] b = a.GetCustomAttributes(true);
foreach(Attribute c in b)
{
Console.WriteLine(c);
}
}
}
}
[AttributeUsage(AttributeTargets.All)]
class vijay : System.Attribute
{
string s1,s2;int i1;
public int mukhi;
public override string ToString()
{
return s1+" " + s2+" " + i1 + " " + mukhi;
}
public vijay(string s ,int i)
{
s1=s;i1=i;
}
public string sonal
{
get { return s2; }
set { s2 = value; }
}
}
[vijay("hi1",10, sonal = "bye1",mukhi = 200 )]
class yyy
{
[vijay("hi2",100, sonal = "bye2",mukhi = 2000 )]
public void abc() {}
[vijay("hi3",1000, sonal = "bye3",mukhi = 2 )]
public void pqr() {}
}
Output
yyy
hi2 bye2 100 2000
hi3 bye3 1000 2
The object m looks like Type. As explained earlier, we are calling a function called GetMethods which returns
an array of MethodInfo's. a loops through each one. We have two methods and the foreach gets executed twice.](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-220-2048.jpg)
![Once for abc and then for pqr. The GetCustomAttributes also exists in a MethodInfo class that returns an array
of objects representing our attributes. We iterate through each, displaying what the ToString function returns.
As we have only one attribute per function, the second for each gets executed only once.
a.cs
using System.Runtime.InteropServices;
using System;
using System.Reflection;
class zzz
{
public static void Main()
{
Type m;
m = typeof(yyy);
System.Console.WriteLine(m.Name);
foreach(MethodInfo a in m.GetMethods())
{
object [] b = a.GetCustomAttributes(true);
foreach(Attribute c in b)
{
if ( c is vijay )
Console.WriteLine(c);
}
}
}
}
[AttributeUsage(AttributeTargets.All)]
class vijay : System.Attribute
{
string s1,s2;int i1;
public int mukhi;
public override string ToString()
{
return s1+" " + s2+" " + i1 + " " + mukhi;
}
public vijay(string s ,int i)
{
s1=s;i1=i;
}
public string sonal
{
get { return s2; }
set { s2 = value; }
}
}
[vijay("hi1",10, sonal = "bye1",mukhi = 200 )]
class yyy
{
[vijay("hi2",100, sonal = "bye2",mukhi = 2000 )]
public void abc() {}
[vijay("hi3",1000, sonal = "bye3",mukhi = 2 )]
public void pqr() {}
}
There is no change at all in the output. A function can be decorated with as many attributes as you like. We
would like to filter out certain attributes.
a.cs
using System.Runtime.InteropServices;
using System;
using System.Reflection;
class zzz](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-221-2048.jpg)
![{
public static void Main()
{
Type m;
m = typeof(yyy);
System.Console.WriteLine(m.Name);
foreach(MethodInfo a in m.GetMethods())
{
object [] b = a.GetCustomAttributes(true);
foreach(Attribute c in b)
{
if ( c is vijay )
Console.WriteLine(c);
}
}
}
}
class vijay : System.Attribute
{
public override string ToString()
{
return "vijay";
}
}
class vijay1 : System.Attribute
{
public override string ToString()
{
return "vijay";
}
}
class yyy
{
[vijay()]
public void abc() {}
[vijay()]
[vijay1()]
public void pqr() {}
}
Output
yyy
vijay
vijay
We have two attribute classes vijay and vijay1. The function pqr has been decorated with 2 attributes whereas
abc with only one. However we do not see vijay1 in the output as the 'c is vijay' makes the if statement true only
for the attribute vijay and not vijay1. For the function pqr GetCustomAttributes returns an array of size two, but
the if statement is true only for one of them, the one with the attribute name vijay. This is because of the 'is'.
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
if ( a is yyy)
Console.WriteLine("a yyy");
xxx b = new yyy();
if ( b is xxx)
Console.WriteLine("b xxx");](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-222-2048.jpg)
![if ( b is yyy)
Console.WriteLine("b yyy");
int d = 10;
if ( d is yyy)
Console.WriteLine("b yyy");
}
}
class xxx
{
}
class yyy : xxx
{
}
Output
a yyy
b xxx
b yyy
We would like to know the data type of an object at runtime. C# offers you a keyword 'is' that lets you check the
data type of an object. a looks like yyy and 'is' results in true. B looks like xxx but is initialized to a new yyy.
Thus it doubles up for a yyy and a xxx resulting in the next two is's returning true. D is an int and not a yyy, so
the last 'is' is false.
Attributes Revisited
Positional parameters are a must whereas names parameters are optional. Attribute parameters can be a bool,
byte, char, short, int, long, float and double. These are the simple types that C# supports. Other data types are
string, enums, objects arrays etc.
Attribute usage has a position parameter which specifies the elements where the attribute can be used. The
default is All. It also has one named parameter called AllowMultiple.
a.cs
using System.Runtime.InteropServices;
using System;
class zzz
{
public static void Main()
{
}
}
[AttributeUsage(AttributeTargets.All)]
class vijay : System.Attribute
{
public vijay(string s)
{
}
}
class yyy
{
[vijay("hi")][vijay("hi1")]
public void abc() {}
}
Compiler Error
a.cs(18,15): error CS0579: Duplicate 'vijay' attribute
By default we cannot use the same attribute twice on any entity.](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-223-2048.jpg)
![a.cs
using System.Runtime.InteropServices;
using System;
class zzz
{
public static void Main()
{
}
}
[AttributeUsage(AttributeTargets.All,AllowMultiple=true)]
class vijay : System.Attribute
{
public vijay(string s)
{
}
}
class yyy
{
[vijay("hi")][vijay("hi1")]
[vijay("hi2") , vijay("hi3")]
public void abc() {}
}
We get no error as by default the AllowMultiple named parameter has a value of false. If we set its value to true,
we are allowed to use multiple attributes on any entity. The above two forms are similar and either one can be
used. Attribute permits us to set declarative information for various program entities for use by someone else at
run time.
Conditionals
a.cs
using System.Diagnostics;
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
class yyy
{
[Conditional("vijay")]
public void abc()
{
Console.WriteLine("abc");
}
}
When we run the above program we get no output at all. In other words the function abc does not get called at
all. This is inspite of writing a.abc().
a.cs
#define vijay
using System.Diagnostics;
using System;
class zzz
{
public static void Main()
{](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-224-2048.jpg)
![yyy a = new yyy();
a.abc();
}
}
class yyy
{
[Conditional("vijay")]
public void abc()
{
Console.WriteLine("abc");
}
}
Output
abc
Any line beginning with a # is read by the C# pre-processor, a program that starts before the C# compiler starts.
It has words like #define which creates a variable or word called vijay. In a programming language, a variable
has to have a value, but in the preprocessor scheme of things, it may/maynot have a value. However if we do not
give it a value, like in this case, the variable is only set to have been defined or created. Anything in [] brackets
is an Attribute class. Earlier we had not created a variable vijay and hence the entire code of abc was left out of
the executable file. Not only that, but all calls to function abc were eliminated from our code. All of this by
defining or not defining a variable vijay. This is what we passed as the attribute to Conditional. We can create
functions that are omitted during compilation depending upon a preprocessing symbol.
When we write code, we add a lot of code for debugging purposes. This code is to help the programmer debug
code. After a function works, it is error free, we do not require any of this debugging code. One way to
eliminate this debugging code is by making the functions conditional. The resulting code is called a 'Retail
build' and the debugging version, obviously a 'Debug build'. A Retail build is much smaller in size and
obviously much faster. The #define has to be at the start of code.
Another way of achieving the same result is by eliminating the #define from the code and creating a
preprocessor symbol as an option to the compiler.
>csc a.cs /d:vijay
The compiler option is /d and the colon is part of the syntax. Following the colon is the name of the
preprocessor symbol. In this case, it is vijay. We’ve discussed preprocessors in one of the earlier chapters.
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
class yyy
{
[Obsolete]
public void abc()
{
Console.WriteLine("abc");
}
}
Compiler Error
a.cs(7,1): warning CS0612: 'yyy.abc()' is obsolete
Output](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-225-2048.jpg)
![unsafe public void abc()
{
int *i ; int *j ; int *k;
int l=1,m=2,n=3;
i=&l; j=&m; k=&n;
Console.WriteLine((int) i + " " + (int) j + " " + (int) k);
pqr(l,m,n);
Console.WriteLine(l + " " + m + " " + n);
}
}
Output
1243460 1243456 1243452
1243424 1243436 1243432
1243428 47645012
1243432 3
1243436 2
1243440 0
1243444 1243508
1243448 1243544
1243452 3
1243456 2
1243460 1
1243464 1243452
1243468 1243460
1243472 1243484
1 2 3
Would it not be a great idea to be able to display whatever is there on the stack. Between variables there are
some gaps and the above program is displaying the contents of the stack. The variable q makes the loop go on
12 times and each time we increment p1 by 4. P1 is pointing to x the first parameter on the stack. Would it not
aid understanding if Microsoft explained what all it pushes on the stack. One of the most common hacking
exploits on the net is called a stack overflow.
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
public class yyy
{
unsafe public void pqr( int *a)
{
Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]);
}
unsafe public void abc()
{
int [] a = new int[3];
a[0] = 10; a[1] = 2; a[2] = 30;
pqr(a);
}
}
Compiler Error
a.cs(20,1): error CS1502: The best overloaded method match for 'yyy.pqr(int*)' has some invalid
arguments
a.cs(20,5): error CS1503: Argument '1': cannot convert from 'int[]' to 'int*'](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-237-2048.jpg)
![We were trying to pass an array as a parameter to a function. Unfortunately C# has some other views.
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
public class yyy
{
unsafe public void pqr( int *a)
{
Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]);
}
unsafe public void abc()
{
int [] a = new int[3];
a[0] = 10; a[1] = 2; a[2] = 30;
fixed ( int *i = a) pqr(i);
}
}
Output
10 2 30
The keyword fixed removed the error. An array in memory can be moved around by C#. We would like C# to
keep it fixed in memory. The keyword fixed guarantees that C# will not move it around in RAM for the duration
of the program. The only reason C# moves objects in memory is to speed up execution of the program. The
keyword fixed as part of syntax wants the () .Within them you can create a variable in our case i which we
initialize to a C# object, an array a. The scope or lifetime of this array is in the next statement i.e. the invocation
of function pqr. In case you have more lines you can use the {}. Even though a now is a pointer to an int, the
notation a[0] refers to the first member. The name of an array tells us where the array starts in memory.
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
public class yyy
{
unsafe public void pqr( int *a)
{
Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]);
a[0] = 100 ; a[1] = 40; *(a+2) = 33;
}
unsafe public void abc()
{
int [] a = new int[3];
a[0] = 10; a[1] = 2; a[2] = 30;
fixed ( int *i = a) pqr(i);
Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]);
}
}](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-238-2048.jpg)
![Output
10 2 30
100 40 33
Changing anything through a pointer changes the original. Whenever we write a[1]= , we are changing the first
member of the original array. a[1] is only a notation, it actually get converted to *(a+1). Thus we are changing
the original members of the array. Also the array grows upward in memory.
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
public class yyy
{
unsafe public void pqr( int *a)
{
Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]);
for ( int i = 0 ; i<= 2000; i++)
*(a+i) = i*10;
}
unsafe public void abc()
{
int [] a = new int[3];
a[0] = 10; a[1] = 2; a[2] = 30;
fixed ( int *i = a) pqr(i);
Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]);
}
}
Please do not run the above program as the error would be unpredictable. The reason being that we are writing
beyond the bounds of the array. The array a created in the function abc has only 12 memory locations allocated
for it. We are writing the next 2000 * 4, 8000 memory locations. The error on your machine will be very
different form another machine. This unpredictability is why pointer errors are hard to catch, and your boss
would thus not like you to use a pointer while programming.
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
struct aaa
{
public int i,j;
}
class yyy
{
unsafe public void abc()
{
aaa *a;aaa b;
b = new aaa();](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-239-2048.jpg)
![public int i,j;
}
class yyy
{
unsafe public void abc()
{
aaa *a;aaa b;
b = new aaa();
a = &b;
Console.WriteLine((int) a);
b = new aaa();
a = &b;
Console.WriteLine((int) a);
}
}
Output
1243468
1243468
We have one object b that we have initialized twice with new. Each time we get the same memory location as
the address of b in memory will remain the same.
Unions
a.cs
using System.Runtime.InteropServices;
class zzz
{
unsafe public static void Main()
{
yyy a = new yyy();
a.i = 65536+512+3;
System.Console.WriteLine(a.i + " " + a.j + " " + a.k);
int *i1;short *j1;byte *k1;
i1 = &a.i; j1 = &a.j ; k1 = &a.k;
System.Console.WriteLine((int)i1 + " " + (int) j1 + " " + (int) k1);
System.Console.WriteLine(sizeof(yyy));
}
}
[StructLayout(LayoutKind.Explicit)]
struct yyy
{
[FieldOffset(0)] public int i;
[FieldOffset(0)] public short j;
[FieldOffset(0)] public byte k;
}
Output
66051 515 3
1243476 1243476 1243476
4](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-241-2048.jpg)
![Let us understand what a union is all about. We are printing the addresses of i, j and k and lo and behold they
are all the same. In a union all the members begin at the same place and that is why their addresses were similar.
If we understand pointers, understanding unions then becomes a piece of cake as we can visually see the
addresses.
a.cs
using System.Runtime.InteropServices;
class zzz
{
unsafe public static void Main()
{
yyy a = new yyy();
a.i = 65536+512+3;
System.Console.WriteLine(a.i + " " + a.j + " " + a.k);
int *i1;short *j1;byte *k1;
i1 = &a.i; j1 = &a.j ; k1 = &a.k;
System.Console.WriteLine((int)i1 + " " + (int) j1 + " " + (int) k1);
System.Console.WriteLine(sizeof(yyy));
}
}
[StructLayout(LayoutKind.Explicit)]
struct yyy
{
[FieldOffset(0)]
public int i;
[FieldOffset(0)]
public short j;
[FieldOffset(10)]
public byte k;
}
Output
66051 515 0
1243468 1243468 1243478
11
a.cs
using System;
class zzz
{
public static void Main()
{
yyy a = new yyy();
a.abc();
}
}
class yyy
{
unsafe public void abc()
{
short i = 512+3;](https://image.slidesharecdn.com/c-obasico-160921154511/75/C-o-basico-242-2048.jpg)
More Related Content
C# o basico
- 1. Contents Authors Introduction Section I 1. GettingStarted 2. Namespaces 3. Constructors and Destructors 4. Components and Databases 5. Web Enabling Data 6. Miscellaneous Section II 7. Function Overloading and Inheritance 8. Modifiers 9. Virtual Functions - new, override 10. Properties and Indexers 11. Interfaces and Structures 12. Operator Overloading 13. Collection Objects 14. Attributes, The Reflection API and Conditionals 15. Unsafe code
- 2. Authors Vijay Mukhi (vmukhi@vsnl.com)is one of the pioneers of the Indian Infotech Industry. For years, he has been the first to teach the emerging technologies in India thus ensuring that India always has people trained in technologies that the world requires. Vijay has written over 80 books on computers and programming over the last eight years on subjects ranging from C, C++ (The Odyssey Series) to Animation and Networking to ERP. His newly released book ‘Java-Servlets-JSP’ has covered many aspects of Java in the most simplified form. Vijay abhors complexities and hence his books showcase the most difficult concepts explained through small programs, thereby giving a good understanding. Microsoft's .Net technologies is what Vijay is now focussing on and he aims at writing volumes on it. Sandeep Shanbhag (sandy.shanbhag@vsnl.com) is a Chartered Accountant by profession, a technical and financial writer by choice. Under the aegis of Wonderland Investment Consultants, he has hitherto been writing regular columns for professional journals, websites and newspapers. Starting out with the aim of leveraging technology to his domain knowledge of finance, this is his first stepping stone. Sonal Mukhi (svmukhi@yahoo.com) is a freelance programmer having a widespread exposure to computer techniques and languages. Sonal has done groundbreaking work on various Internet Technologies like Java, ActiveX, Perl and more. She has co-authored a few books on computer programming too.
- 3. Introduction Welcome to thefirst installment on a series of 12 volumes dedicated to the latest technology to hit the third rock from the sun. But before diving in and getting both feet wet, a wee bit of background. Microsoft has always had a habit of doing things in style. Whether it was the basic yet robust Disk Operating System (DOS) or the (user-friendly?) Windows Operating System, Microsoft has always managed to rewrite the rules in the computer world. In fact it won't be exaggeration to say that today it is thanks to the efforts of Microsoft and the vision of its founder, Bill Gates, that it is software and not machines, that occupy centrestage. But then that is another story altogether. Notwithstanding its hegemony over operating systems, it must be said that Microsoft took its time to wake up to the call of the Internet. Netscape was way ahead in the race, and Internet Explorer was a distant follower, almost nowhere in the reckoning. Even as users had begun to write off Microsoft as a non-starter for the web, it has gone and done what he does best ---pounced back with technology that is more cutting edge than its peers. Although, Netscape isn't history as yet, most people would rather use a browser that works better with the latest versions of Windows. Internet Explorer literally chooses itself. Then of course there are other utility tools like Excel, Word and Power Point - all Microsoft-owned and near-indispensable. And now Microsoft has come out with Microsoft.Net, a project until recently called Microsoft's Next Generation Windows Services architecture. The objective is the integration of the Internet - to offer software - like Word and Excel - through your browser on the computer as well as on a range of devices such as the mobile phone and the personal digital assistant (Palm Pilot etc). The grand design, as it were, is to change the very nature of the way people work presently. Today we generally use one computer per person. Soon, all we shall be using would be only the Internet. For all our needs. In other words, whenever you switch your computer on, you would be connected directly to the Internet and not your hard disk for even the most rudimentary of tasks. The task is enormous: every package now on offer will need additional code (even recoding). In the new regime, Microsoft has got Visual Studio .Net which can enable even the most uninitiated to design complex websites provided of course one is web and design savvy. And all of this will be made possible through a new programming language called C# (pronounced C-Sharp). Why C-Sharp? When people talk of computer programming languages, there is always a debate on which one is better. C++, Java, Perl, PHP3… the arguments and counter-points can be endless. However, it is our considered belief that that C# would be in a sense better than the rest. And there is a reason for this. Normally, whenever a new product is introduced, it betters the technologies of its ilk. For existing technologies evolve with their own pros and cons. Therefore, in 1995, when Sun introduced Java, it took the best from amongst all languages and added current technology. Now Microsoft has done the same. Its taken the best features from C++ and Java and morphed them into this new programming language called C#. Imagine those who worked on Java when it was launched six years ago. Today a CV boasting of a six-year experience in Java means the maximum possible. Simple arithmetic - Java was launched only in '95. Similarly, if you get into C# today, only a Microsoft engineer could have more working knowledge of it! Therefore, we don't think we need to elaborate further on the weight that a CV with C# exposure would command! Think for yourself. You may know of people who have struck lucky in getting a job. But when it comes to keeping it, Lady Luck is powerless.
- 4. Indeed, technology isruthless, in that, it breaks across all age and seniority barriers. That's the reason why even a teenager could have as much experience with a new technology as a 40-year-old. Unfair, but true. Who should read this book? Anyone wanting to be on the cutting edge of new technology. The book is written assuming no prior programming knowledge on the part of the readers. However, we make certain other assumptions that we shall explain now. Many a times, on a clear night when we look up at the stars, we can't help but wonder whether there is intelligent life out there - or are they just like us? For we don't believe that intellect is a quality that we are born with. In fact we are going to share a secret with you. It is the secret of success given to us by an old gypsy woman. This magic mantra has worked with many and we assume you would also use it in your life. In fact this is the only assumption we make in this book. The secret is in persistence - nothing in the world can take place of persistence. Talent will not, nothing is more common than unsuccessful people with talent. Genius will not, unrewarded genius is almost a proverb. Education will not, the world is full of educated derelicts. Persistence and determination alone are omnipotent. We hope you agree with the old gypsy apply the maxim while learning C#. Now, the mere fact that you are reading this means that you are interested in learning something new. However, there is a great deal of difference between a person who wants to read a book and the person who just wants a book to read. We hope you belong to the former category. Though we attempt to infuse a certain degree of humour in our writing, please do not buy this book for its jokes. This is no coffee table book or even bedtime reading. This is a book, a medium, which we hope will do the serious job of teaching the reader a new programming language. And how do we propose to go about it? Just like when you eat, no matter how delicious the food in front of you is, you have to consume it, one mouthful at a time. You just cannot eat it all at once, can you? Similarly, when learning, it is always better to take small steps - one concept at a time. Sometimes, we even repeat the same concept several times, in different ways. For we firmly believe that if you do the little things well enough, big things will come to you asking to be done. Incidentally, the entire C# software is available on the Microsoft site, downloadable free of charge. Detailed instructions are given on the site along with a set of FAQs. Well, that's that. We agree, it is tough to climb the ladder of success, especially if you're trying to keep your nose to the grindstone, your shoulders to the wheel, your eye on the ball and your ear to the ground. Precisely the reason why some people do not recognise opportunity - it often comes disguised as hard work. This book presents you with precisely such an opportunity. We hope you grab it with both hands! Acknowledgements I wish to thank a number of people who gave me support, new ideas and inspiration through the process of writing this book. First and foremost, thanks to Manish Jain, BPB publication for publishing the book. Special thanks to my co-authors, Sonal and Sandeep, who have put in their very best in the work assigned to them as without them and their efforts the book would have never seen the light of day. Thanks to my cover designers, Altaf Hemani and Kishore Rohra, for designing the cover in a very short notice.
- 5. Thanks to ManishPurohit for putting in all the time he had to verifying the code with the explanation, testing all the applications in the book and updated them to work under the Release version of the product. To, Pradeep Mukhi and Shivanand Shetty, who made it simple for me and my co-authors to come up with the book. A long list of friends and my family need a mention here for their patience and cooperation on this book while it was being written. -Vijay Mukhi
- 6. 1 Getting Started C# ispronounced as "C sharp". It is a new programming language that enables programmers in quickly building solutions for the Microsoft .NET platform. Today, one cannot, just cannot, afford to ignore C#. It is our considered opinion that it holds immense promise and we are going to try our best, through this book, to help you realize its potential. Be assured, we are not going to teach you just another programming language. It is our intention to help you apply C# in practical situations, to actually implement your newly acquired knowledge on the Net. With this brief introduction, let us embark on a path that will take you to new adventures in the world of Internet. In this chapter, we will get you started with C# by introducing a few very simple programs. For remember, even a journey of a thousand miles must begin with a single step. We assume that you have no prior knowledge of any programming language. But before we get ensnared in the fascinating world of C#, let's make a directory where we will save all our work. In order to do so, click on Start, Programs, then go to Accessories and select Command Prompt (Windows 2000) or the MS-DOS Prompt as it is called in Windows 98. Once you are at the command prompt create a directory called csharp (md csharp) and change to this directory (cd csharp). Now type the command 'edit a.cs', which will open the MS-DOS editor - the world's simplest editor. C:csharp>edit a.cs Yes, we very well understand how you must be yearning to write your first C# program and get it working. But before we do that, there are certain intricacies that you must understand. What a.cs refers to is called the filename or program name. Here we have named our file or program a.cs. Why a.cs? Well, before we began writing this book, we consulted a renowned astrologer who predicted that if we named our first file a.cs then great luck would be showered on us. Not wanting to quarrel with the stars, we named our file a.cs. But you are free to go ahead and call your file any name you want. But then do so at your own risk! Remember, forewarned is forearmed! Jokes aside, 'cs' is the extension used for C# files. They say of all the things you wear, your expression is the most important. Notwithstanding this, one does look more dapper in a suit rather than a vapid shirt and trousers. Similarly, though it is not mandatory to provide the extension 'cs', you can make a filename seem more impressive by giving it an extension. To reiterate, you could have given the extension say 'ws' too; it does not matter. But absent minded as we are, it is more prudent to give appropriate extensions while naming files. As the first step, we will understand the basic structure of a C# program. a.cs class zzz { } Here we start with the word class zzz followed by open and close curly braces. A class is nothing but a collection --- a collection of everything that the programming language contains. It is like a packet or a container, which can hold anything. Hence everything in a C# program must be enclosed within a class. We have named our class zzz, again you could have named it anything else but if you would rather follow our naming convention (for reasons well amplified above!), name it zzz.
- 7. Now for thepart that you've been eagerly waiting for! In order to execute the program, go to the File menu, and click on Exit. You will get a dialog box asking you whether you want to save the file or not, say yes. Now that we have typed and saved our file we need to execute it. The compiler creates executable code. The command used to call the C# compiler is csc followed by the program name. Since our program name is a.cs, the command csc a.cs will call the compiler. A compiler is a program which understands the C# programming language. Thus the word class is part and parcel of the C# language. Microsoft lets you freely download the C# compiler from their web site : http://msdn.microsoft.com/library/default.asp. Select .Net Framework SDK under .NET Development. Choose the Download option to download the sdk which is around 127 MB large. Install the product on your machine cause if you don’t, none of the following programs will work. Also, Internet Explorer 5.5 and Microsoft Data Access Components(2.7) must be installed prior to installing the sdk Once done, type the command as follows: C:csharp>csc a.cs You will see the following output on your screen in the dos box. Microsoft (R) Visual C# Compiler Version 7.00.9254 [CLR version v1.0.2914] Copyright (C) Microsoft Corp 2000-2001. All rights reserved. error CS5001: Program ‘a.exe’ does not have an entry point defined Just as the excitement was beginning to grow, our program returns with an error message. Don't worry, occasional failure is the price of improvement. The error message starts with an error number CS5001 followed by a cryptic message, which we do not understand. We are aware of the fact that everything has a beginning and an end. Similarly, a C# program also has a start and an end. Ah! Now you realize why the error occurred. We forgot to tell C# where to start executing our program from. This starting point is also called an entry point. You can specify the entry point by adding static void Main() to your program, just as we have done below. a.cs class zzz { static void Main() { } } Compile the above code giving the command as csc a.cs. Voila! Now no errors. The compiler will now generate an exe file called a.exe. Giving the command dir at the command prompt will prove the same. On keen observation you will notice that among the 2 files listed, there is a file by the name a.exe. Simply type 'a' at the command prompt, your program will now be executed! C:csharp>a The program will run but shows no output on our screen. But, at least we get no errors. The words, static and void, will be explained to you a little later, in the right perspective. Thus if you had felt the beginnings of a massive headache, you can breathe easy! Anything followed by '(' and ')' brackets is called a function. So, it is obvious that Main is nothing but a function. Here we are creating a function called Main. It is followed by the '{' and '}' curly braces. Note that the letter M in Main is capital. Thus C# requires a function called Main, which is the first function that will be executed. Failure to do so will result in an error. Ergo,
- 8. whenever you seea word beginning with an open '(' and close bracket ')', C# and most other programming languages call it a function. The { signifies the start of a function and the } signifies the end of the function. The guys who designed the language decided to use {} braces instead of start and end. When you tell people that you are learning a programming language, you are actually learning to use {} braces to specify the beginning and end of a function. These rules have to be remembered by rote. You have no choice. Now we are ready to add some code in our program. In order to do so, add the line WriteLine("Hell"), just as we have done below. a.cs class zzz { static void Main() { WriteLine("Hell") } } Oops! The astrologer had promised showers of luck! Even a drizzle seems far away! Executing the above program results in the following error: Compiler Error a.cs(5,18): error CS1002: ; expected The error message begins with the file name, a.cs followed by round brackets containing the line number and the column number, where the error occurred. The compiler informs us that it found an error on line number 5 and column number 18 and it expects a ;. As the cliché goes, when the going gets tough, the tough get going. So we shouldn't lose heart as yet; let's understand why this error occurred. Look at WriteLine within the brackets; doesn't it ring a bell? Isn't WriteLine a function too? But here we do not have the curly braces following it. This is because here we are not creating a function like we created Main. Then what is it that we are doing? We are calling a function called WriteLine, which has already been created. The error says ';' expected. Though it is obvious to us that the statement has ended, unfortunately for you and me, C# isn't so smart. It requires a semi-colon to tell it loud and clear that the statement has ended. Merely pressing enter does not suffice. Though not so for other programming languages, in C# it is mandatory to use a semi-colon. Alas! Each programming language has its own rules! At this juncture, an intelligent question would be - But why semi-colon? Why not any other punctuation mark? We don't know for sure but perhaps the developers of C# were asked to select a character that would indicate the end of a statement. The non-compromising species that we are, they could not arrive at a consensus. Result? Grandma's recipe of eene meene mina mo! When they stopped, their finger was pointing at the semi-colon, hence they selected the same. Thus rules are rules; as preposterous as they may sound, they must be followed. Add the semi-colon and execute the program. Also languages like ABAP/4 from SAP ends lines with a dot (full stop). Thus we expect to understand programming in English. Every language expects an end of statement/command terminator. Every language expects a symbol to denote when the user has finished saying something. In C# it is a ; , other languages have their own symbols. Remember that the statement or function WriteLine was written on line number 5 hence the error reported line number 5. a.cs class zzz { static void Main() { WriteLine("Hell"); }
- 9. } Compiler Error a.cs(5,1): errorCS0103: The name 'WriteLine' does not exist in the class or namespace 'zzz' Another error! We are extremely disappointed, it just doesn't seem our day. However we firmly believe that you may be disappointed if you fail, but are surely doomed if you don't try. And we don't want to be doomed do we? So let's keep our chin up and carry on. But you know what's most irritatingly bothersome? Why are all error messages so difficult to understand? Well, with the experience that we have gained over the years we have learnt that if error messages could be understood then we wouldn't get them in the first place! In any case, to be truly candid, the error occurred because we pulled a fast one on you! We are calling a function WriteLine but in fact no such function exists. The accurate name of the function is not WriteLine but System.Console.WriteLine. Microsoft is known to fancy big names and here is one more example. In any case, let's execute the program. a.cs class zzz { static void Main() { System.Console.WriteLine("Hell"); } } Output Hell Finally, no errors! Surely you feel blessed! It displays the word 'Hell', and we suspect that's exactly what you are going through right now. But if you follow our programs step by step we assure you that heaven is not too far away. Now remove all the extra spaces and 'enters' from your program, just as we have done below and compile the program again. a.cs class zzz {static void Main() {System.Console.WriteLine("Hell");}} Output Hell You will notice that doing so does not give any errors. Just like the previous program, it displays 'Hell'. The C# compiler is not affected by it. But using spaces and 'enters' definitely make your program neater and thereby more readable. The effect is especially appreciated in large programs and more so when someone else has to go through them. Having said that, anyway, the first thing that C# compiler does is removes all the spaces and enters from the program code you have written. In the next program, we have called WriteLine function twice. a.cs class zzz { static void Main() { System.Console.WriteLine("Hell"); System.Console.WriteLine("Bye"); } }
- 10. Output Hell Bye On executing thisprogram, 'Hell' and 'Bye' are displayed on two separate lines. Here, we are not required to give anything that has the 'enter' effect, WriteLine automatically prints on a new line each time. Which simply means you can call a function as many times as you like. In the next illustration, let's understand how functions are called and created. Here we are calling a function abc(). a.cs class zzz { static void Main() { abc(); } } Compiler Error a.cs(5,1): error CS0103: The name 'abc' does not exist in the class or namespace 'zzz' On executing this program you will get, what else, but an error. Peter Drucker, the famed management guru had once said that the better a man is, the more mistakes he will make, for the more new things he will try. So there you go - next time you encounter an error, simply tell yourself that you have just become better at whatever you are doing. In any case, the error says that abc does not exist. Here we are calling a function called abc(), but where is abc() defined or created ? It is not a function that has been provided by C# to us free of charge. It is our own homegrown function that we are calling. The lesson here is that we cannot call a function without first creating it. So, to rectify this error we will first create a function abc. Our next example demonstrates this. a.cs class zzz { static void Main() { abc(); } static void abc() { System.Console.WriteLine ("Hell"); } } Output Hell In the function abc, we have included only one statement- WriteLine within the curly braces. The '{' and '}' braces indicate the beginning and the end of this function. Alternatively, a function can contain millions of lines of code that will be executed when the function is called. Since everything is contained in a class, our function abc is also created within the class zzz but outside Main. But the function is called from Main. On executing the program, 'Hell' is displayed. This is because we have included the code for 'Hell' to be displayed in the function abc. Thus, when the control reaches the line abc(); it searches for that function and executes the code within that function. We will explain static and void later as promised. You can call as many functions as you like from your program. But you must remember to separate each one with a semi-colon. The next program illustrates this. a.cs class zzz
- 11. { static void Main() { abc(); pqr(); abc(); } staticvoid abc() { System.Console.WriteLine ("I am ABC"); } static void pqr() { System.Console.WriteLine ("I am PQR "); } } Output I am ABC I am PQR I am ABC At first the function abc is called, then pqr and then again we are calling abc. On executing this program 'I am ABC', 'I am PQR' and 'I am ABC' will be displayed. This is because we have included the code for these lines to be displayed in the respective functions. In the following program we are calling the function pqr from abc and not from Main. a.cs class zzz { static void Main() { abc(); } static void abc() { pqr(); System.Console.WriteLine ("I am ABC"); } static void pqr() { System.Console.WriteLine ("I am PQR "); } } Output I am PQR I am ABC In the function abc, we are first calling pqr and then displaying 'I am ABC' using the WriteLine function. Hence, first 'I am PQR' is displayed and then 'I am ABC'. Thus, this program demonstrates how functions can be called from other functions. Now that we have created our own functions abc and pqr, we have an intuitive understanding of how C# created the function System.Console.WriteLine. The next program uses the printing or formatting capabilities of the WriteLine function. a.cs class zzz { static void Main()
- 12. { System.Console.WriteLine("100 {0}",100); } } Output 100 100 Thezero in the curly braces means that after the first comma there is some value and that it should display this value. You cannot differentiate between the two 100's. The {0} is replaced with 100, the number, which follows the first comma. If you don't like the number 100, use the number 420 instead. We won't mind - at least it's something that some of you can easily identify with! The program below is simply an extension of the above. a.cs class zzz { static void Main() { System.Console.WriteLine("100 {0},{1}",100,200); } } Output 100 100,200 Here the {0} is replaced with 100 and {1} is replaced with 200. The comma (,) separates the two numbers. Thus {0} means the first number and {1} means the second. C# likes to count from zero and not one. a.cs class zzz { static void Main() { ii; ii=20; System.Console.WriteLine ("{0}",ii); } } Compiler Error a.cs(5,1): error CS0201: Only assignment, call, increment, decrement, and new expressions can be used as a statement a.cs(5,1): error CS0103: The name 'ii' does not exist in the class or namespace 'zzz' a.cs(6,1): error CS0103: The name 'ii' does not exist in the class or namespace 'zzz' a.cs(7,33): error CS0103: The name 'ii' does not exist in the class or namespace 'zzz' Experience is knowing a lot more of things you shouldn't. But now that you have decided to embark upon this book, let's see what you should know!! Executing this program results in a large number of errors as shown above. Let's understand the rationale behind it. Here we have included a strange word --- ii. This word ii is being given a value of 20 as ii=20. But C# is a shy program. It does not speak to strangers! Here ii is nothing but a stranger to C#. C# does not know who or what ii is. You can't just write words that you feel good about. So, in order to rectify this error we must tell C# who ii is. Our next program will demonstrate how this can be done. a.cs class zzz { static void Main()
- 13. { int ii; ii=20; System.Console.WriteLine ("{0}",ii); } } Output 20 Notethat in this program we have added the word int before ii. The word int indicates that ii is an integer or a number. Each time we create our own word like ii, C# wants to know what we will store in this word. We will understand this better and in totality in just a little while. Here we are initializing ii to 20 or giving it a value of 20 by writing ii = 20. Why 20? Maybe because we are feeling very youthful today! Following this we have the WriteLine function. Now, it is a known fact that jaggery is a good substitute for sugar. Similarly, in C# you can substitute a number with a word. So, in WriteLine we have used the word ii instead of a number. The word ii gets replaced with the value 20. So, on executing the program, the number 20 is displayed. You may be wondering as to why should you ever use a word when you can use a number directly. Our next program will enlighten you on this. a.cs class zzz { static void Main() { int ii; ii=20; System.Console.WriteLine ("{0}",ii); ii=30; System.Console.WriteLine ("{0}",ii); ii=ii+10; System.Console.WriteLine ("{0}",ii); ii=ii+1; System.Console.WriteLine ("{0}",ii); ii++; System.Console.WriteLine ("{0}",ii); } } Output 20 30 40 41 42 The first three lines of this program are identical to those of our previous one. Thus in the first, the WriteLine function will display the number 20. Thereafter, by saying ii=30 we are initializing the word ii to a value 30. In effect, we are changing the value of ii from 20 to 30. So, WriteLine will now display 30. For conceptual understanding, we earlier introduced ii as a word. However, in the software world, it has another name; it is actually called a variable. Thus, a variable is a word whose value varies or changes. ii initially had a value 20 that changed to 30. Coming back to what the word int means- when you say int, int means integer or number. When we say int ii, it means that the variable ii will store a number. ii could also be used to store the letters of the alphabet like the names of people. It could also store a date. But here we wanted ii to store a number. So, we have to tell C# in advance as to what the variable is going to store. Hence we say int ii. C# understands the word int as int is a part of the C# programming language.
- 14. The variable iistarted with the value 20 and is now holding a value 30. In the next line we have ii=ii+10. The 'equal to' sign makes this statement look complicated. To avoid confusion, always start by looking to the right of the 'equal to' sign. To the right of the equal to sign we have ii+10. Since ii is holding the value 30, ii+10 is read as 30+10, which evaluates to 40. Hence ii=ii+10 will now be read as ii=40. This value 40 is given to the variable on the left-hand side. Now that ii has a value 40, WriteLine will display 40. Similarly, we say ii=ii+1. Here the value of ii is incremented by one. And the new value of ii, which is 41 will be displayed by WriteLine. In the next line we have ii++. Note that ii=ii+1 and ii++ do the same thing. They both increment the value of ii by 1. Hence WriteLine will now print the number 42. This is a big problem with programming languages. As there are many ways to skin a cat, there are also many ways to increase the value of a variable. So ii = ii + 1 and ii++ do the same thing. A programmer thus has the choice to use either of the two ways to do the same thing. There you go ---as you can see, first time programmers have unfortunately a lot to learn. In daily life, a hundred different people can do the same chore in a hundred different ways. Similarly, in programming there is more than one way of doing a particular thing. Programming becomes a problem because of redundancy. Since there are two ways of doing the same thing you have to learn them both and that can be very confusing. Of course, it is at your discretion to choose the method you are comfortable with. Henceforth, even though there may be many other ways of doing a particular thing, we will teach you only one method that we find appropriate. A variable is a word that stores a value. This value can be changed at will by the programmer. Wherever we can use a number, we can also use a variable. The next program may seem alien to you. This is because it speaks the truth. If it finds that a condition is true it outputs true and if it finds the condition to be false it outputs false. Very unlike some of you, who would do just the opposite! a.cs class zzz { static void Main() { System.Console.WriteLine ("{0}", 5 > 2); System.Console.WriteLine ("{0}", 5 < 2); System.Console.WriteLine ("{0}", 2 < 2); System.Console.WriteLine ("{0}", 2 <= 2); System.Console.WriteLine ("{0}", 2 != 2); System.Console.WriteLine ("{0}", 2 !=3); } } Output True False False True False True Let's understand this program line by line as usual. Here the first line says WriteLine("{0}", 5 > 2); Is 5 > 2 ? Yes. So the condition 5 > 2 evaluates to true. Hence the {0} in WriteLine is replaced with True and the WriteLine function displays True. 5 > 2 is called a condition. A condition is like a question which has to result in a yes or no or a true or false. In this case the number 5 is greater than the number 2 and hence the function evaluates to true. C# understands the words true and false. In the second WriteLine we have the condition 5 < 2. Is 5 < 2? No. So the condition 5 < 2 evaluates to false and hence WriteLine displays False. Similarly, the next condition 2 < 2 is not true and hence False is displayed.
- 15. In the nextstatement we have 2 <= 2. Here the first sign being '<' it is first checked whether 2 < 2? No, 2 is not less than 2. So then the second sign '=' is checked i.e., whether 2 = 2? Yes. So the condition 2 <= 2 evaluates to true and WriteLine displays True. <= is thus two conditions in one . In the next WriteLine we have the condition 2 != 2. '!=' means 'not equal to'. But 2 is equal to 2, hence the condition evaluates to false and False is displayed. The comparison is false, therefore the condition is false. In the last statement we have the condition 2 != 3. Is 2 != 3? Yes 2 is not equal to 3, so the condition evaluates to true and True is displayed. This True and False is a special data type in C#. It is called 'Bool' or boolean derived from Boolean algebra. a.cs class zzz { static void Main() { bool ii; ii=true; System.Console.WriteLine ("{0}", ii); ii=false; System.Console.WriteLine ("{0}", ii); } } Output True False In our previous programs we used the data type int for the variable ii. This meant that ii could store integers or numbers. Similarly, you can now initialize ii to either true or false. This is possible by using the data type bool for ii. A data type means the type of data a variable can hold. Therefore, here we are saying bool ii. In the first case we are initializing ii to true. So in WriteLine, {0} is replaced with true. In the second case ii is initialized to false and {0} is replaced with false. In a gist, we now know that variables can be either bool or logical or they can contain the words True or False. Also, variables can also contain numbers. Let's see how C# distinguishes between data types. a.cs class zzz { static void Main() { bool ii; int jj; ii=10; jj=false; } } Executing the above program gives the following errors: Compiler Error a.cs(6,4): error CS0031: Constant value '10' cannot be converted to a 'bool' a.cs(7,4): error CS0029: Cannot implicitly convert type 'bool' to 'int' If you want the rainbow, you gotta put up with some rain. So let’s understand the reason behind the errors. Here C# is internally saying that it distinguishes between data types. In that sense C# is very petty about what values you give to variables. Bool can't mix with int and vice versa. Since ii is a bool you can't initialize it to 10, which is a number. C# is very strict about this. And because jj is an int you can't initialize it to false. So you have to be very careful about how you initialize your variables. After all it doesn't make sense to ask someone who is good
- 16. at driving toteach computers and vice versa. Everyone has his or her place in life. Similarly, variables also have their place. Hence a variable declared as a bool can only have values true or false. Similarly, int can have only numbers or integers, it cannot take true or false values. Our next program is similar to one of the previous programs. a.cs class zzz { static void Main() { bool ii; ii=6 < 7; System.Console.WriteLine ("{0}", ii); } } Output True Here ii is a variable of type bool. In the next statement we have ii = 6 < 7. As you already know, in such a case you should start by looking to the right of the 'equal to sign'. Because 6 is less than 7 the condition evaluates to true and ii will be initialized to true. Hence {0} is replaced with the value of ii, which is true and True is displayed. C# is called a strongly typed language because you cannot initialize a variable of type int to a variable of type bool. The reason C# is very strict about this is because this system eliminates a large number of sloppy programming errors. Some languages like C on which C# is based on do not care what values you initialize your variables to. C# is stricter than even Java. In some ways, it's like your mother when it comes to telling you what you are doing wrong in life. Thus it is extremely difficult to make dumb errors in C#. Remember when you are writing code in C#, the compiler is always peering down your shoulder making sure you do not get away with any errors. If Statement So far the code that we have written is rather useless. This is because it always gets executed. Life is enjoyable only because it is unpredictable. We don't know what tomorrow is going to bring. Variety is the spice of life! Similarly, we would like to write computer programs, which add spice to programming. We want to write programs that behave differently depending upon different situations or circumstances. And this can be achieved with the help of the if statement. So fasten your seatbelts and get set to ride! The next few programs will demonstrate the usefulness and application of the if statement. a.cs class zzz { static void Main() { if ( false ) System.Console.WriteLine ("Hi"); } } Compiler Warning a.cs(6,1): warning CS0162: Unreachable code detected In this program we have included the word 'if' followed by false in round brackets. This is the if statement and its syntax. Syntax is the grammar in writing. Thus we have no choice but to abide by the rules. It is called a statement because anything that C# recognizes is called a statement. When you run this program, you realize
- 17. that there isno output. So this program will simply display nothing. Now you know why we get the above warning as our code will never get executed. Let's understand the rationale behind it. The if statement lets you include decision making in your program. It decides whether to execute the next line or not. When the if statement evaluates to false then the next line is ignored. The if statement brings with it the power to decide whether certain code should be executed or not. The following program will make this concept clearer. a.cs class zzz { static void Main() { if ( false ) System.Console.WriteLine ("Hi"); System.Console.WriteLine ("Bye"); } } Output Bye The if statement looks at the immediate next line, it doesn't look at the line after that. Since the if statement influences only one line, you will see only Bye displayed and not Hi. Same warning again. But if you want both the statements to be affected by the if then enclose them within curly braces. This is illustrated below. a.cs class zzz { static void Main() { if ( false ) { System.Console.WriteLine ("Hi"); System.Console.WriteLine ("Bye"); } } } Here we have included both the lines of code within the braces. Now the if statement will affect both the lines of code. The condition being false, nothing is displayed. Thus, the if statement gives us the option to execute or not to execute a certain piece of code. If we always use false then the code will never be called. But what did we tell you about conditions? They return either true or false. So, in the next program we have 3 > 8 as the condition. Since 3 is not greater than 8 the condition evaluates to false. Since there are no curly braces, only the next line is affected by the if statement. Hence only Bye is displayed. a.cs class zzz { static void Main() { if ( 3 > 8 ) System.Console.WriteLine ("Hi"); System.Console.WriteLine ("Bye"); }
- 18. } Output Bye Let's look atanother variation of this program. Now, the interesting part is that wherever you can use a condition you can also use a variable which is of type bool, which we know evaluates to either true or false. a.cs class zzz { static void Main() { bool ii; ii=true; if ( ii ) { System.Console.WriteLine ("Hi"); System.Console.WriteLine ("Bye"); } } } Output Hi Bye Here the variable ii is declared as a bool and then initialized to true. In the next line we have if (ii). The variable ii holds the value true and hence the if condition evaluates to true. The condition being true both Hi and Bye are displayed. Note that here both the statements are included in the curly braces resulting in both the statements being affected by the if statement. Thus the if statement is affecting a block of statements. Declaration is another way of saying that we are creating a variable. In the following program we have an if with the else. If the if condition evaluates to true then the statement following it is called but if it evaluates to false then the else is called. a.cs class zzz { static void Main() { if ( false ) System.Console.WriteLine ("Hi"); else System.Console.WriteLine ("Bye"); } } Output Bye Here, since the condition evaluates to false the else is called and hence Bye is displayed. Thus the additional else statement specifies a statement that is executed when the condition is false. This construction covers all possibilities, as a condition can be either true or false. In an 'if-else' construct one of them have to be executed. Computer programs are said to be made intelligent because of the if statement. The more the use of the if statement, the more intelligent your program becomes. The if statement is one of the main building blocks of any programming language. Thus all programming languages have to have a if statement. Loops
- 19. The if statementis the cornerstone of programming because it lends intelligence and a decision making power to the language. The second important constituent of any programming language is a looping construct. In a program many times you would need to repeat instructions. In C#, the for statement is one form of a loop that lets you repeat statements. However, as we already know, a statement can also be a block of statements, thus it also allows repetition of multiple statements. Our next program explains the for loop. a.cs class zzz { static void Main() { int ii; for ( ii = 1; ii <= 5; ii++) System.Console.WriteLine ("Hi {0}", ii); } } Output Hi 1 Hi 2 Hi 3 Hi 4 Hi 5 The for has 2 semicolons as part of its syntax or rules. The statement up to the first semicolon is executed only once. For the first time and only the first time ii is initialized to 1. Remember up to the first semicolon the statement is executed only once. The statement enclosed within the first and second semicolon is a condition. The condition checks whether ii <= 5 evaluates to true. Since this condition evaluates to true, the statement within the open and the close braces gets executed. If the condition evaluates to false, the statement is ignored and the loop terminates. The variable ii has a value 1 which is less than 5, so System.Console.WriteLine will be called which displays 'Hi 1' as the value of ii is 1. After the statement gets executed, the last part of the for i.e. from the second semicolon to the closing bracket gets executed. ii++ will increase the value of ii by 1, making it 2. The condition is checked again, is 2 <= 5. The answer here is true, so 'Hi 2' is displayed. And this roller coaster goes on till the condition is false. When ii has the value 6, the condition checked is, is 6 <= 5. The answer being false, the for terminates. This is how the for statement enables the repetition of code. Our next example will further help to illustrate this. a.cs class zzz { static void Main() { int ii; for ( ii = 1; ii <= 5; ii++) System.Console.WriteLine ("Hi {0}", ii); System.Console.WriteLine ("Hi {0}..", ii); } } Output Hi 1 Hi 2 Hi 3 Hi 4 Hi 5 Hi 6..
- 20. In this programwe have two WriteLine statements. The for loop follows the same rules as the if statement. Thus in absence of curly braces the for loop will affect only the immediate next statement. Therefore, the for loop will print numbers from 1 to 5 along with hi. The moment the for loop terminates, ii with the dots will print Hi 6... This goes to prove that when ii has a value six, the for loop will terminate. The following program demonstrates how the for loop enables repetition of multiple statements. a.cs class zzz { static void Main() { int ii; for ( ii = 1; ii <= 5; ii++) { System.Console.WriteLine ("Hi {0}", ii); System.Console.WriteLine ("Hi {0}..", ii); } } } Output Hi 1 Hi 1.. Hi 2 Hi 2.. Hi 3 Hi 3.. Hi 4 Hi 4.. Hi 5 Hi 5.. Here, both the WriteLine statements are enclosed within curly braces. Therefore, both the statements are affected by the for loop. Hence, in this case, each number is displayed twice along with Hi, once without the dot and once with the two dots. Similar to the for loop is the while loop. a.cs class zzz { static void Main() { int ii; ii=1; while ( ii <= 5 ) { System.Console.WriteLine ("Hi {0}", ii); } } } The while loop takes a condition, hence the variable ii is initialized to 1 before entering the loop. The condition checks whether ii <= 5 evaluates to true. Currently the value of ii is 1. The condition evaluates to true and the statement within the curly braces is executed. System.Console.WriteLine is called with Hi and the value of ii. Note that here we are not changing the value of ii within the loop. Since the value of ii remains 1 the condition always evaluates to true and the loop will go on forever, indefinitely. We can't have anything go on forever, can we? There has to be a stop to it somewhere! Our next program will resolve this problem. a.cs class zzz {
- 21. static void Main() { intii; ii=1; while ( ii <= 5 ){ System.Console.WriteLine ("Hi {0}", ii); ii++; } } } Output Hi 1 Hi 2 Hi 3 Hi 4 Hi 5 This program is similar to the previous one. The only change that we have made is that we added the line ii++; thus for the first time Hi 1 is displayed. Then the moment ii++ is encountered the value of ii is incremented by 1, making it 2. Then the condition, 2 <= 5, is checked. The condition being true, once again we enter the loop and Hi 2 is displayed. This will go on until ii becomes 6. Once ii is 6 the condition evaluates to false and the loop terminates. Your mind may ponder over the question, "Should I use a for or a while?' The for loop is similar to the while loop. To answer your question in the most simple manner "On Mondays, Wednesdays, Fridays use for and on Tuesdays, Thursdays, Saturdays use while. Sundays we assume no one writes code". Alternatively "Toss a coin, heads use while, tails don't use for" ;-) In other words, it is at your discretion to use the one you are comfortable with. Once again C# offers you multiple ways of doing the same thing. The Return statement Let's understand the return statement with our next example. a.cs class zzz { static void Main() { int ii; ii = abc(); System.Console.WriteLine("hi {0}",ii); } static int abc() { System.Console.WriteLine("abc"); return 100; } } Output abc hi 100 In this program, at first, ii is declared as a variable of type int. Then we start executing the line ii=abc(); Doesn't the right hand side of the statement ring a bell? Aren't we calling the function abc() ? Hence at this point the control passes to the function abc().
- 22. In the functionwe have the WriteLine statement, which prints 'abc'. Thereafter, we have a return statement. It says return 100. In effect, the function abc() is returning a value 100. Thus the statement ii = abc(); will now be read as ii=100. Now that ii has a value 100, {0} in WriteLine is replaced with 100 and in turn 100 is displayed. a.cs class zzz { static void Main() { int ii; ii = abc(); System.Console.WriteLine("hi {0}",ii); } static void abc() { System.Console.WriteLine("abc"); return 100; } } Compiler Error a.cs(6,6): error CS0029: Cannot implicitly convert type 'void' to 'int' a.cs(12,1): error CS0127: Since 'zzz.abc()' returns void, a return keyword must not be followed by an object expression Executing the above program results in errors. Here we have static void abc(). What static means, we will explain a little later. It is with the intention to enable you to learn better, so please bear with us. The word void means nothing. So by saying void abc() we are saying that the function abc() does not return any value. Functions may/maynot return values. Earlier abc returned an int. When a function returns void as in this case, we are saying that abc cannot return a value. But in the function code we have written return 100. Therefore, because of this contradiction we get the error. Note that in the previous program we did not get an error. This was because we had said static int abc(). Here int signifies the return type of the function. Since the function returns a number we used int. In our next example we have made only one addition i.e. we have added System.Console.WriteLine("Finish"); after the return statement. a.cs class zzz { static void Main() { int ii; ii = abc(); System.Console.WriteLine("hi {0}",ii); } static int abc() { System.Console.WriteLine("abc"); return 100; System.Console.WriteLine("Finish"); } } Compiler Warning a.cs(13,1): warning CS0162: Unreachable code detected Output abc hi 100
- 23. On executing thisprogram, you will find that the output is the same as the previous program. To your amazement you do not see the word 'Finish' displayed. No, it is not because C# doesn't like you. The reason is that anything after the return statement is ignored. Thus, you may have a hundred WriteLine statements after the return statement but they will all be ignored. Simply stated, no lines after return statement will get called. That is why the C# compiler, courteous as it is, gives you a warning and not an error. More on data types....... You have already learnt about two data types, int and bool. Our next program introduces a new data type 'string'. a.cs class zzz { static void Main() { string s; s= "hell"; System.Console.WriteLine(s); } } Output hell In this program, the variable s is declared as a string. This implies that s will store mainly letters of the alphabet. Then s is initialized to hell, which is nothing but a collection of the letters of the alphabet. Note that all strings have to be enclosed within double inverted commas. System.Console.WriteLine displays the value of the variable s, which is 'hell'. Earlier we wrote a string in double inverted commas directly. However, this time we are using a variable name instead. Now we know the first parameter to the WriteLine function is a string. We earlier named our variable ii and now we have called it s. Actually, we name our variables depending upon the time we write our programs. You decide your own rules for naming variables. Anything you enclose in double quotes is called a string. System.Console.WriteLine is smart enough to display strings, bools and int. Let's look at another variation of the above program. a.cs class zzz { static void Main() { string s; s= "hell"; System.Console.WriteLine( "{0}", s); } } Output hell This program is exactly like the previous one, the only difference being that we have used {0} instead of writing only s. Here the {0} is replaced with the value of s, which is 'hell'. Thus, using the {0} is preferable as it understands a large number of data types and how to display their values. Consolidating, our next example incorporates all the data types that we have learnt so far.
- 24. a.cs class zzz { static voidMain() { string s; int ii; bool jj; ii=10; jj=false; s="hell"; System.Console.WriteLine("{0} {1} {2}", ii, jj, s); } } Output 10 False hell Here the variable s is declared as a string. Then ii is declared as int and jj is declared as bool. In the next three statements we are initializing each of the variables. ii is initialized to 10, jj is initialized to false and s is initialized to hell. Now, with the help of a single WriteLine statement we are displaying the values of all the variables. {0} is replaced with 10, {1} is replaced with False and {2} is replaced with hell. This goes to prove that all the data types can be displayed together, in a single WriteLine statement. Thus, C# allows all the data types to co-exist in harmony. Now only if the people of our country could do the same! Passing parameters to functions By now you are familiar with functions and how functions are called. The next program illustrates how parameters are passed to functions. a.cs class zzz { static void Main() { abc(10,false,"hell"); } static void abc(int i, bool j, string s) { System.Console.WriteLine("{0} {1} {2}", i,j,s); } } Output 10 False hell We have again used the System.Console.WriteLine function to display values of variables or merely display text onto the screen. To have it print something on to the screen, we had to give it the things that we wanted printed. These things are nothing but parameters. We don't pass things to functions; we pass parameters to functions. So far we never created our own functions with parameters. In this program, we are calling the abc function with three things, with three parameters. The first is a number, the second is a logical value and the third is a string. So, we are passing the values 10, false and hell to the function abc(); These values must be stored somewhere, but where? When we create the function abc we have to state the names of three variables along with their respective data types. This is because the values that we pass will be stored in these variables. Hence we have the variables i, j and s. These are then displayed using WriteLine. Therefore, it will output 10, False and hell. This is how parameters are passed to functions in C#. Remember you decide what names to give to variables. Parameters passed to functions are also variables. a.cs class zzz { static void Main() {
- 25. abc(10,false); } static void abc(inti, bool j, string s) { System.Console.WriteLine("{0} {1} {2}", i,j,s); } } Compiler Error a.cs(4,1):error CS1501: No overload for method 'abc' takes '2' arguments On compiling this program you will encounter the above error. Here we are calling abc with only two parameters, 10 and false. Whereas the function abc is actually created with three parameters. We are passing an erroneous number of parameters, hence the error. Lesson? One must pass the same number of parameters that the function has been created to accept. A mismatch between the number of parameters being passed and those being accepted will definitely assure you of an error. Thus as before, C# does a large number of error checks on your code. For, if it allowed the above function call to go ahead, what would the value of the third parameter s be? Now change the order of the values that are being passed. Your abc function should look like this- abc("hell",10, false); a.cs class zzz { static void Main() { abc("hell",10,false); } static void abc(int i, bool j, string s) { System.Console.WriteLine("{0} {1} {2}", i,j,s); } } Compiler Error a.cs( 5,1): error cs1502: The best overloaded method match for 'zzz.abc(int, bool, string)' has some invalid arguments a.cs( 5,5 ): error cs1503: Argument '1': cannot convert from 'string' to 'int' a.cs(5,12): error cs1503: Argument '2': cannot convert from 'int' to 'bool' a.cs(5,15): error cs1503: Argument '3': cannot convert from 'bool' to 'string' On executing this program you will be faced with the above errors. These errors have resulted due to a data type mismatch. It is somewhat like putting a round pole in a square peg! How is it going to fit? While calling the function abc() the first parameter that we are passing is the word "hell". But the function abc() has been created to accept the first parameter as an int. Remember we told you, not so long ago, that C# distinguishes between data types. Thus you can't store a string in a variable of type int. Similarly, the value 10 cannot be stored as a bool and false cannot be stored as a string. Therefore, not only should the number of parameters being passed and accepted be the same but also their data types must match. Like oil and water do not mix, in the same way you cannot give C# an int when it wants a string. As said earlier, some languages are more forgiving then C# due to which the programmer makes more mistakes in them. a.cs class zzz { static void Main() { abc(); } }
- 26. class yyy { static voidabc() { System.Console.WriteLine("abc"); } } Compilation Error a.cs(5,1): error CS0103: The name 'abc' does not exist in the class or namespace 'zzz' Seems like we are hooked on to errors! In this program we have created another class called yyy. We can have as many classes in one .cs file as we like. This program now has two classes, zzz and yyy. The class zzz has a function called Main. Main denotes the starting point of our code and as such is the first function to be called. The class yyy contains a function called abc. Here we are calling the function abc by saying abc(); in class zzz. But class zzz does not have any function by the name abc. Merely giving abc(); encourages C# to assume that abc() exists within the class zzz. But our abc function is contained in yyy. Hence we get an error as we are trying to call a function that exists in another class. But we are adamant! We want to use the abc function that yyy has. It's human tendency to want things that others have! So, in the next program we are calling the function abc in the class yyy by writing yyy.abc(); a.cs class zzz { static void Main() { yyy.abc(); } } class yyy { static void abc() { System.Console.WriteLine("abc"); } } Compilation Error a.cs(5,1): error cs0122: 'yyy.abc() is inaccessible due to its protection level. When we say yyy.abc(); why the dot? yyy is the name of the class and abc() is the name of the function . Each of these names can be as large as you want, so to separate them a dot is used. Thus when you want to access a function that belongs to another class you can do so by specifying the class name and function name separated by a dot. The dot is like the semicolon. The designers of the language wanted some character as a separator between class name and function name, they chose the dot, they could have also chosen the semicolon again. You will now realize why we say Console.WriteLine. Obviously, it means that Console is a class and within the class Console there is a function called WriteLine. But to your dismay, on executing this program you still get an error. Well, few people get what they want, however fewer still want what they get! You may want a particular thing but you will not get it unless the other party gives you the permission to take it, use it or share it. Your only other option is to put up a fight! We get an error here because we haven't used the word public. The whole idea behind C# is its usability on the net. And the only way you can use it on the net is by having security precautions. So in C# the default rule is - you can't use anything unless explicitly granted permission. Next question. How do you grant the necessary permission?
- 27. Being its non-violentself, C# grants permission by using the word public. When we say public, we mean the world at large. So by starting with the word public we are saying that the whole world is now allowed to use this function as we are explicitly granting you rights. If you don't use the word public it will give you an access error. To rectify the error, add the word public just as we have done below. a.cs class zzz { static void Main() { yyy.abc(); } } class yyy { public static void abc() { System.Console.WriteLine("abc"); } } Output abc Finally, we have the correct code! See, one of life's greatest ironies is the fact that when you finally master a tough job, you make it look easy!! By using the word public in front of the function abc, we can now call it from zzz by specifying yyy.abc(); Now that the function is called, WriteLine displays 'abc'. Our next example will enhance your understanding further. a.cs class zzz { static void Main() { yyy.abc(); abc(); zzz.abc(); } public static void abc() { System.Console.WriteLine("abc in zzz"); } } class yyy { public static void abc() { System.Console.WriteLine("abc"); } } Output abc abc in zzz abc in zzz
- 28. Now we goa step further. In the above program we have two abc functions, one in class zzz and one in class yyy. If you want to call the one in yyy then you say yyy.abc(); but if you want to call the one in zzz then you say zzz.abc() or abc(). The function abc(), by itself, will ask which class is it? Since abc() is in zzz itself, C# assumes it to zzz.abc(); By implication, if the function exists within the same class, it is optional to preface the function with the name of the class. When you execute this program, yyy.abc() will call the abc function in class yyy and WriteLine will display 'abc'. Thereafter, both abc() and zzz.abc() will call the abc function in class zzz. And in each case WriteLine will display 'abc in zzz'. Thus if you do not preface the function name with the name of the class, C# will add the name of the class in which you are calling the function. In our case it is zzz. 2 Namespaces Normally, students have an implicit trust in their teachers. For they know that if the blind lead the blind, both shall fall into the ditch. However, our philosophy is that a good teacher has to be an even better liar!! In accordance with this belief, initially we told you that WriteLine is the name of a function. Then we told you that the name is not WriteLine, it is System.Console.WriteLine. But even that's not true. Now comes the moment of truth. Console is actually the name of a class. Yes, trust us! There is no crying wolf here. The class Console has a function called WriteLine. Hence the name now becomes Console.WriteLine. However, that leaves out the word System. Now what does System mean? Well, a number of functions like WriteLine are available in C#. Some functions will let you print, some will enable writing of data to disk and others will let you create graphics. The problem that we are posed with is - how does one remember which function satisfies what purpose? Wouldn't it make sense if we logically grouped similar functions together? So, Microsoft thought that all functions that can write to the screen could be made part of one class. All functions that let you work with pictures could be part of another class. But even then, you will have too many functions in one class. So they thought of having a single higher logical group. Such that all the functions that have anything to do with the screen, i.e. whether you are drawing pictures or writing text, be grouped once again into a higher body. Thus all classes that deal with interacting with a database could go into a group called Data. The second problem that we are posed with is that of name clashes. What do we mean by that? Now, nothing can stop me from creating my own class called Console and include a function called WriteLine in it. But how will C# know which Console am I referring to? The one that we created ourselves, or the one that Microsoft has already created. In order to resolve these problems Microsoft decided to take classes and put them into namespaces. What is a namespace? It is simply a word. Thus we can logically group everything as per namespaces. From the above explanation you would have by now guessed that System is nothing but a namespace. The following programs will help make this concept crystal clear. a.cs class zzz {
- 29. static void Main() { yyy.abc(); abc(); zzz.abc(); } publicstatic void abc() { System.Console.WriteLine("abc in zzz "); } } namespace vijay { class yyy { public static void abc() { System.Console.WriteLine("abc"); } } } Compiler Error a.cs(5,1): error CS0246: The type or namespace name 'yyy' could not be found (are you missing a using directive or an assembly reference?) In the above program, the only change that we have made is that we have now included the class yyy in a namespace called vijay. On doing so you will realize that the same program that worked earlier doesn't work anymore. This is because yyy is put in a namespace vijay. A namespace is nothing but a word with an open and close brace. The entire class is enclosed within the namespace. If you want to access a function belonging to class yyy from another class then the only way to do so is by saying vijay.yyy.abc(); Thus you specify the namespace first, then the name of the class followed by the name of the function, each of them separated by dots. Thus like Urdu, we read anything from the right and not the left. We start with the name of a function, then the name of the class and whatever is left is the namespace. Here zzz has not been given a namespace. If you don't specify a namespace then by default the class is included in a global namespace. Now change yyy.abc(); to vijay.yyy.abc() and watch the error disappear. a.cs class zzz { static void Main() { vijay.yyy.abc(); abc(); zzz.abc(); } public static void abc() { System.Console.WriteLine("abc in zzz "); } } namespace vijay { class yyy { public static void abc() { System.Console.WriteLine("abc"); } }
- 30. } Output abc abc in zzz abcin zzz We bet your faces are now beaming! Seems like the secret of happiness is not in doing what one likes to do but in liking what one has to do. We had to get rid of the error which we have succeeded in doing. The error has disappeared; the program executes as advertised and it generates the same output as it did previously. Thus all the classes and functions created by Vijay can be included in a namespace called vijay. If Sonal creates a namespace by her name then she can include all the functions and classes created by her in the namespace sonal. Thus there will be no name clashes at all. These namespaces that are created by us are called user-defined namespaces whereas System is a pre-defined namespace. Thus System is a namespace in which a class called Console was created, which contained a function called WriteLine. Thus the namespace concept allows us to create logical groups. So all xml related classes can be in a namespace called xml, web can be in a web namespace and so on and so forth. But the only problem now is that when you start writing code you have to specify the namespace first, then the class name followed by the function name. Well, everything is available but for a price! Our consolation is that it is a very small price to pay. a.cs namespace mukhi { class zzz { static void Main() { vijay.yyy.abc(); abc(); zzz.abc(); mukhi.zzz.abc(); } public static void abc() { System.Console.WriteLine("abc in zzz "); } } } namespace vijay { class yyy { public static void abc() { System.Console.WriteLine("abc"); } } } Output abc abc in zzz abc in zzz abc in zzz In the above program, we have two classes zzz and yyy. The class zzz is included in a namespace called mukhi and the class yyy is included in a namespace called vijay.
- 31. So when wesay abc(); in Main, Main is in zzz, so you are actually writing mukhi.zzz.abc(); This is because C# will automatically expand it since the function abc is available within the same class. Hence it is at your discretion as to how you want to write it. You can say abc(), zzz.abc() or mukhi.zzz.abc(); finally they all expand to namespace.classname.function name. C# adds the name of the namespace and the name of the class even if you do not specifically write it. The concept of namespaces is not very difficult to understand. It allows for a hierarchical grouping of classes. It tells us which classes are logically grouped. It also avoids classes from having the same name. a.cs class zzz { static void Main() { Console.WriteLine("abc in zzz "); } } Compiler Error a.cs(5,1): error CS0246: The type or namespace name 'Console' could not be found (are you missing a using directive or an assembly reference?) If we do not enclose our class in a namespace, it becomes part and parcel of the global namespace ‘ ‘. This namespace does not contain a class called Console. We had mentioned earlier that the class Console is contained in the namespace System. We do not want to preface the Console class with the namespace System each and every time. The only reason being that our fingers will wear out typing the word System over and over again. So C# lets us use a shorthand by means of which we avoid the pain of having to keep on writing the name of a namespace over and over again. a.cs using System; class zzz { static void Main() { Console.WriteLine("abc in zzz "); } } Output abc in zzz The secret here is not in doing great things, but doing a small thing in a great way. We get no error simply by employing the word using which is part of the C# language. All that using does is whenever it sees only the name of a class, it goes and adds (in this case) the word System. Thus we do not have to write the word System over and over again. This works the way shorthand does. a.cs using System; class zzz { static void Main() { yyy.abc(); } } namespace vijay { class yyy { public static void abc() {
- 32. Console.WriteLine("abc"); } } } Compiler Error a.cs(6,1): errorCS0246: The type or namespace name 'yyy' could not be found (are you missing a using directive or an assembly reference?) Now we get an error for yyy and not for Console as the yyy class belongs to the vijay namespace and not the global namespace. Because of the using keyword, C# adds the namespace System to yyy yielding System.yyy.abc and realizes that System does not contain a class called yyy. Hence the error. a.cs using System; using vijay; class zzz { static void Main() { yyy.abc(); } } namespace vijay { class yyy { public static void abc() { Console.WriteLine("abc"); } } } Output abc The error vanishes as C# first tries System.yyy.abc gets an error, then tries vijay.yyy.abc and is successful. Thus by having two using's we do not have to write the namespaces vijay or System ever again. a.cs using System; using mukhi; using vijay; namespace mukhi { class zzz { static void Main() { yyy.abc(); abc(); zzz.abc(); zzz.abc(); } public static void abc() { System.Console.WriteLine("abc in zzz "); } } } namespace vijay { class yyy
- 33. { public static voidabc() { System.Console.WriteLine("abc"); } } } Output abc abc in zzz abc in zzz abc in zzz We can have as many using's as we like and the compiler will try each one in turn. If none of them match we will receive an error. In this case it will try 3 times with System, mukhi and vijay and if none match, you will get an error. a.cs using System.Console; class zzz { static void Main() { WriteLine("abc in zzz "); } } Compiler Error a.cs(1,7): error CS0138: A using namespace directive can only be applied to namespaces; 'System.Console' is a class not a namespace After the word using you can only write the name of a namespace. System.Console is a namespace class combination which is not allowed. Building Hierarchy In C# you organize classes using namespaces. Now let's discover the extent we can go to as far as organizing classes. a.cs class zzz { static void Main() { mukhi.vijay.yyy.abc(); } } namespace mukhi { namespace vijay { class yyy { public static void abc() { System.Console.WriteLine("abc"); } } } }
- 34. Output abc In this programwe have a namespace within a namespace i.e. within the namespace mukhi we have another namespace vijay. Thus namespaces are 'hierarchical'. If you want to access the function abc in yyy you have to specify it in the form- namespace.classname.functionname. So, the qualified name is now mukhi.vijay.yyy.abc(); Once the function is called, WriteLine will display 'abc'. In order to differentiate between the various names separated by dots, always read backwards. Reading backwards, the first is the function name then the class name and the names thereafter will all be namespaces. Alternatively, you can directly specify the namespace as mukhi.vijay, as we have done below. This program generates the same output as previously, it prints abc. a.cs class zzz { static void Main() { mukhi.vijay.yyy.abc(); } } namespace mukhi.vijay { class yyy { public static void abc() { System.Console.WriteLine("abc"); } } } Output abc Here we have a single namespace by the name mukhi.vijay. The name mukhi.vijay is actually a shortcut for defining a namespace named mukhi that contains a namespace named vijay. In this program, we have only two namespaces. But you can expand it further to include a number of namespaces depending upon the level of hierarchy required by your program. We can liken this to an organization. Let's consider mukhi to be the name of the company. Within that you have a sub-company or a division called vijay, which creates it own classes. As such the level of hierarchy can be expanded. Before you understand the next program let's address a simple question. Why do you use classes? Classes are used because they offer a large number of functions. You don't use classes because of the variables that you can create within them; you use classes for the functions that they provide. Remember, you call a function using the form -namespace.classname.functionname. File Operations a.cs class zzz { static void Main() { File.Copy("c:csharpa.txt","c:csharpb.txt",true); } } Compiler Error
- 35. a.cs(5,1): error CS0246:The type or namespace name 'File' could not be found (are you missing a using directive or an assembly reference?) Our next program will enlighten you on something most sought after - The art of Copying. Ah! Finally something of interest! This program introduces the 'Copy' function. It allows you to duplicate a file. File is a class and it has a function called Copy, which is static. The first parameter 'a.txt' is the source file i.e. the file, which we want to duplicate. The second parameter 'b.txt' is the destination file i.e. the file that we want to copy to. Note that you must specify the entire path for the file name. The last parameter 'true' implies that if the file exists then it will be overwritten. If the file does not exist it will be created and contents of the source file will be copied onto it. And just when you thought you had mastered the art of copying the program returns with an error message. The error says C# does not know what File.Copy is. The problem is that the name of the namespace is System.IO. So you have to specify the namespace too. Add the namespace and execute the program. a.cs class zzz { static void Main() { System.IO.File.Copy("c:csharpa.txt","c:csharpb.txt",true); } } The program does not generate any compilation errors. Create a file called a.txt with some text before you run this program. Execute this program and then open the file 'b.txt'. Finally, the task has been accomplished! You now have the contents of a.txt copied into b.txt. Our next program introduces another function called 'Delete'. a.cs class zzz { static void Main() { System.IO.File.Delete("c:csharpa.txt"); } } The above program takes the name of a file as the parameter. This function will remove the file specified from disk. Give the dir command at the command prompt and you will find that the file has been deleted. Every language will offer you millions of such functions like copy and delete. These functions were always available, but C# has gone one step further and made these functions a part of a Class. They are now part of a Namespace. Hence it becomes easier to categorize functions. It is but a question of detail whether you should or should not categorize them.
- 36. 3 Constructors and Destructors New Let'sconsider the following program. a.cs class zzz { static void Main() { yyy.abc(); } } class yyy { public void abc() { System.Console.WriteLine("abc"); } } Compiler Error a.cs(5,1) : error CS0120: An object reference is required for the nonstatic field, method or property 'yyy.abc()' This program contains one class called yyy which has a function called abc. In Main() we are using the syntax yyy.abc() to call the abc function as we did earlier. Within the abc function we have the code for the function abc that will print the string "abc". On compiling this program you will get an error as incomprehensible as ever! But how is it that when we ran this program earlier we didn't get an error? If you are still bewildered, wake up and smell the coffee! Didn't you notice we removed the word 'static' while saying public void abc(). Hence we get an error. In our earlier programs when we wrote the function abc we had written the word static which is missing now. No, we are not going to tell you to add the word static and execute the program. We are not that predictable! On the contrary, we shall do something quite different and interesting. Keeping that in mind let's consider the next program. a.cs class zzz { static void Main() { yyy a; a.abc(); } } class yyy { public void abc() { System.Console.WriteLine("abc"); }
- 37. } Compiler Error a.cs(6,1): errorCS0165: Use of unassigned local variable 'a' Before we look into this program let's get our basics clear. We have often used the statement 'int i' meaning that i was a variable that looked like an int. When we used the statement 'string s', it meant that s was a variable that looked like string. Similarly, in this program we are saying yyy a. This implies that 'a' looks like yyy. What is yyy? It is the name of a class. Here we do not call 'a' a variable, we call it an object. An object and a variable can be used interchangeably. Earlier, whenever we wanted to call a member from a class we would say yyy.abc(); i.e. class name dot function name. But in our current program we are saying a.abc(); We are using the same dot, but now it gives an error saying - 'Use of unassigned local variable'. Note that the word member is analogous with the word function. But things still don't seem any clearer. So, let's go a step further and add another statement a=new yyy(); Match your code with the one below. a.cs class zzz { static void Main() { yyy a; a=new yyy(); a.abc(); } } class yyy { public void abc() { System.Console.WriteLine("abc"); } } Output abc The word or keyword new must be followed by the name of a class; You cannot substitute it with anything else, it must be a class. In our case, we have given the statement as new yyy(). yyy is the name of an existing class. But why have round brackets after the class name? The '(' and ')' brackets are part of the syntax. And you very well know by now that you can't argue with syntax. Thus it is at this point i.e. after saying new, that the object 'a' that looks like yyy is created. We could have also called the object 'a' an instance of the class yyy. Since the class yyy has only one function it will allocate memory for THAT one function ONLY. Now we have an object 'a' that looks like class yyy. Once the object is created, it can be used to access the function from class yyy. Hence, now if we say a.abc() we will not get an error. Thus an object is nothing but an instance or an occurrence of a class. Therefore, 'a' is an instance of the class yyy. This is how you can instantiate an object. In order to create an object you must use the keyword 'new'. Our next program will help you gain a better understanding of this concept. a.cs class zzz { static void Main()
- 38. { int i; i=new int(); } } Atfirst we have int i, meaning i looks like int. Then we have i=new int(); Executing this program will not generate any errors. But so far whenever we used int i we never created the object i using new int(). This is because C# does this internally for us. It saves us the trouble of doing so. Then why doesn't C# do so for all the other objects that we create? This is because C# recognizes only two types of classes. The first type is one that the C# compiler knows of in advance. int, long, bool, and string are classes that fall into this category. These are predefined classes. But we call them data types because in C and C++ they were called data types and C# has a legacy of C and C++. So technically, when we say int i it does i=new int(); internally. The second type of classes that C# recognizes are the ones that we create i.e. user-defined classes. For user- defined classes we have to create objects ourselves. Thus anything other than the basic data-types must be explicitly created. So when we say 'yyy a' we are not creating the object at this point. We are only declaring the object to be of type yyy. It is only when we use the word 'new' that the object is created and memory is allocated for the same. Therefore, when we have our own classes, that's the time we use new. Without new you cannot create an object. Static You are certainly going to benefit by the patience you have shown so far. To find out how, let's follow the next program. a.cs class zzz { static void Main() { yyy a; a=new yyy(); a.abc(); yyy.pqr(); } } class yyy { public void abc() { System.Console.WriteLine("abc"); } public static void pqr() { System.Console.WriteLine("pqr"); } } Output abc pqr In this program we have two functions abc and pqr. It is of significance to note that the function pqr has the word static whereas abc does not. If you want to access the static function pqr you say yyy.pqr() and to access the non static function abc you say a.abc(); You can't do the reverse i.e. you cant say a.pqr() and yyy.abc(). a.cs
- 39. class zzz { static voidMain() { yyy a; a=new yyy(); yyy.abc(); a.pqr(); } } class yyy { public void abc() { System.Console.WriteLine("abc"); } public static void pqr() { System.Console.WriteLine("pqr"); } } Compiler Error a.cs(7,1): error CS0120: An object reference is required for the nonstatic field, method, or property 'yyy.abc()' a.cs(8,1): error CS0176: Static member 'yyy.pqr()' cannot be accessed with an instance reference; qualify it with a type name instead The word 'static' implies 'free'. Static signifies that you can access a member or a function without creating an object. At last you are enjoying the fruits of patience! Observe that the function Main in zzz is static. This is because we are not creating any object that looks like zzz. The crux is that if you don't want to use 'new' and yet use the function then you must make the function static. In both cases a dot is used to reference the function. The only difference is that a static member belongs to the class and as such we don't need to create an object to access it. On the other hand, a non-static member, that is the default, can be accessed only via an object of the class. Thus WriteLine is a static function in Console as we did not create an object that looks like Console to access it. a.cs class zzz { static void Main() { System.Console.WriteLine(yyy.i); } } class yyy { public int i = 10; } Compiler Error a.cs(5,26): error CS0120: An object reference is required for the nonstatic field, method, or property 'yyy.i' Why did we get an error? Think for thinking is the hardest work there is, which is probably the reason why so few engage in it. If you still haven't got it, let us enlighten you. The same rules for static apply to functions as well as variables. Hence we get the above error. a.cs class zzz
- 40. { static void Main() { yyya = new yyy(); yyy b = new yyy(); a.j = 11; System.Console.WriteLine(a.j); System.Console.WriteLine(b.j); yyy.i = 30; System.Console.WriteLine(yyy.i); } } class yyy { public static int i = 10; public int j = 10; } Output 11 10 30 A static variable belongs to the class. Hence if we create a static variable i, no matter how many objects we create that look like yyy, there will be one and only one value of i as there is only one variable i created in memory. Thus we access a static variable by prefacing with the name of the class and not name of object. If the variable is non-static like j then we have to use the syntax as explained earlier i.e. name of object dot name of variable. Thus each time we create an object that looks like yyy we are creating a new/another copy of the variable j in memory. We now have two j's in memory one for a and another for b. Thus j is called an instance variable unlike i. When we change the variable j of a to 11, the j of b remain at 10. Thus functions are created in memory only once, irrespective of the word static. If a class has no instance or non static variables then it makes no sense to create multiple instances of the object as there will be no way of distinguishing between the copies created. Constructors a.cs class zzz { public static void Main() { yyy a; System.Console.WriteLine("Main"); a=new yyy(); } } class yyy { public yyy() { System.Console.WriteLine("yyy"); } } Output Main yyy
- 41. In the aboveprogram we have a class called yyy. We also have a function called yyy which happens to be having the same name as the name of the class. We have a friend named Bunty. Coincidentally, the name of his pet dog is also Bunty! Similarly, it is absolutely legal to have a function by the same name as that of the class. In this case first we see Main and then we see yyy displayed on the screen which means that the function yyy() gets called automatically. Note, we have not called the function yyy explicitly. This happens to be a special function and it is called a 'constructor'. Initially we are saying yyy a. By doing so we are specifying that 'a' looks like yyy. We are not creating an object that looks like yyy. The next statement has System.Console.WriteLine, which will print 'Main'. Thereafter, using new we are creating an object a. It is at this point that C# calls the constructor i.e. it calls the function yyy(). Now you will see 'yyy' displayed. This goes to prove that as soon as an object of a class is created, C# automatically calls the constructor. A constructor gets called at the time of birth or creation of the object. It has to have the same name as the name of the class. a.cs class zzz { public static void Main() { yyy a; System.Console.WriteLine("Main"); a=new yyy(); a.yyy(); } } class yyy { public yyy() { System.Console.WriteLine("yyy"); } } Compiler Error a.cs(8,1): error CS0117: 'yyy' does not contain a definition for 'yyy' Here, we are calling the function yyy using the appropriate syntax i.e. by saying a.yyy(). Now, run the compiler. Baffled by the error? The error says 'yyy' does not contain a definition for 'yyy'. The class yyy does contain a function called yyy which got called in the previous example. Has C# developed amnesia all of a sudden? What went wrong? Well, you cannot call this function using a.yyy() or yyy.yyy() The catch is that when you have a function with the same name as that of the class you cannot call it at all. It gets called automatically. C# does not give anyone the authority to call such a function. It calls this function automatically only at birth. Seems abnormal doesn't it! But what is the purpose of having constructors? A constructor can be used in cases where every time an object gets created and you want some code to be automatically executed. The code that you want executed must be put in the constructor. That code could be anything, it could check for hard disk space, it could create a file or it could connect to the net and bring a file over. What that code will do shall vary from person to person. To understand how and when the constructor gets called, let's take into consideration our next example. Now remove the word 'public' in front of yyy() as we have done below. a.cs class zzz { public static void Main() { yyy a;
- 42. System.Console.WriteLine("hi"); a=new yyy(); } } class yyy { yyy() { System.Console.WriteLine("yyyconst"); } } Compiler Error a.cs(7,3): error CS0122: 'yyy.yyy()' is inaccessible due to its protection level. Obviously, you will get an error. This is because without the word public the function is private property. And when you trespass on private property you have to face the consequences. In our case we are faced with an error. By making the function public every one can use it, it is now becomes public property! If the constructor is private then nobody can create an object that looks like yyy. Do you think constructors can return values? Let's try it out and find out for ourselves. a.cs class zzz { public static void Main() { yyy a; System.Console.WriteLine("hi"); a=new yyy(); } } class yyy { public int yyy() { System.Console.WriteLine("yyy const"); } } Compiler Error a.cs(12,12): error CS0542: 'yyy': member names cannot be the same as their enclosing type Executing the above program generates an error. It says that member i.e yyy cannot be the same as the enclosing type i.e class yyy. Now, that is an error that you certainly didn't expect. Let us analyze why we got this error. Here we are saying public int yyy implying that the function yyy() is returning an int. yyy() is a constructor and is called automatically at the time an object is created. If a constructor is to return a value then to whom should it return the value to? Since it is called automatically, there is nothing that can accept the return value. Thus constructors cannot return values. Also when a constructor gets called, an object is in the act of being created. It has not yet been created. The keyword new first allocates memory for the functions and the variables. After this it calls the constructor. When the constructor finishes, then we say that the object has been created. Thus the constructor has no one to return values to. Now that we know constructors don't return values let's return void instead. a.cs class zzz {
- 43. public static voidMain() { yyy a; System.Console.WriteLine("hi"); a=new yyy(); } } class yyy { public void yyy() { System.Console.WriteLine("yyy const"); } } Compiler Error a.cs(12,13): error CS0542: 'yyy': member names cannot be the same as their enclosing type Though we are returning void, we get the same error. This is because C# is very sure of what it says. When you borrow from other people you do so on the pretext that you will return it. It's a different story that you never do! You rarely mean what you say. But when C# says that constructors cannot return values it means 'constructors cannot return values', not even 'void'. Remember, there is nothing that can accept the return values. Hence even void cannot be accepted. When a function returns a void we mean that it will return no value at all. For a constructor, the word return makes no sense at all. Constructors with parameters Just as we pass parameters to other functions, you can also pass parameters to constructors. a.cs class zzz { public static void Main() { yyy a; System.Console.WriteLine("hi"); a=new yyy("no"); } } class yyy { public yyy() { System.Console.WriteLine("yyy const"); } } Compiler Error a.cs(7,3): error CS1501: No overload for method 'yyy' takes '1' arguments You are already aware of the fact that parameters are passed to functions while calling them. Similarly, we will pass a parameter to the constructor yyy while creating the object a; because it is at this point that the constructor gets called. Hence we are saying a=new yyy("no"). But on compiling this program you get an error. Here, the constructor is being called with a string 'no' as a parameter. But there is no variable in the constructor yyy to store the value 'no'. Add 'string s' in the constructor yyy and watch the error disappear. a.cs class zzz {
- 44. public static voidMain() { yyy a; System.Console.WriteLine("hi"); a=new yyy("no"); } } class yyy { public yyy(string s) { System.Console.WriteLine(s); } } Output hi no At first WriteLine will display 'hi'. Then we have a constructor yyy that takes a string 'no' as a parameter and accepts it in a variable s. Thus the moment the constructor yyy is called 'no' will be displayed. This is because the constructor yyy contains code that will print the value stored in the variable s. This is how constructors with parameters are called. So far we created only one instance of the class yyy. In the following program we are creating two instances of the class yyy, 'a' and 'b'. a.cs class zzz { public static void Main() { yyy a,b; System.Console.WriteLine("hi"); a=new yyy("no"); b=new yyy(); } } class yyy { public yyy(string s) { System.Console.WriteLine(s); } } Compiler Error a.cs(8,3): error CS1501: No overload for method 'yyy' takes '0' arguments While creating the object 'a' the constructor yyy is being passed a parameter 'hi'. In case of the object 'b' the constructor will be called without any parameters. But in our program we have code only for the constructor with parameters. Try executing this program and you will get an error saying that method yyy takes 0 arguments. This is because we do not have code for the constructor without parameters. Let's understand the reason behind the error. In our earlier programs, we did not specify a constructor. A relevant question here would be - how did the objects get created then, without the constructor being called? C# is a Good Samaritan, at that time it inserted a
- 45. free constructor. Itdoes so internally. On its own it inserts a constructor without any parameters, without any code into the class. It looks like this- yyy() { } Point to ponder - in the above program, when we didn't create a constructor without parameters why didn't we get one free? Remember we said C# is a Good Samaritan? And Good Samaritans help the needy. On seeing that we already have a constructor with parameters, C# looks the other way i.e., it takes the free one away. However, it is only when you have no constructor at all that C# melts its heart and gives you one free. Remember, even if it finds that you have even one constructor it will take away the free one. On the assumption that if you can provide one constructor, then with a little effort you can work towards providing the others too! Now the only way to get rid of the error is to add the constructor yourself. a.cs class zzz { public static void Main() { yyy a,b; System.Console.WriteLine("hi"); a=new yyy("no"); b=new yyy(); } } class yyy { public yyy(string s) { System.Console.WriteLine(s); } public yyy() { System.Console.WriteLine("bad"); } } Output hi no bad Here, initially, the two objects 'a' and 'b' are declared. In the next statement we have WriteLine, which will display 'hi'. We are then creating the object a. At this point the constructor with a string as a parameter is called. It will now display the value stored in the variable s which is 'no'. Thereafter, the object b is created. At this point the constructor without the parameters will be called. This constructor contains the WriteLine statement, which will print 'bad'. Here, we have as many constructors as we are calling and hence we do not get any errors. So, essentially, a constructor is a special function that gets called when an object is created. It does not return any values, not even void. As far as parameters go it behaves like a normal function. If no constructors are specified you get one free, otherwise, you need as many constructors as you are calling. Hence in the above case we cannot create an object as new yyy(100) as we do not have a constructor that accepts one int as a parameter. Destructors a.cs public class zzz
- 46. { public static voidMain() { aa a = new aa(); } } public class aa { public aa() { System.Console.WriteLine("Constructor "); } ~aa() { System.Console.WriteLine("Destructor"); } } Output Constructor Destructor A destructor is a function with the same name as the name of a class but starting with the character ~. A constructor gets called at birth whereas a destructor gets called at death. In C# unlike other languages we do not know when an object dies as unlike James Bond, we do not have a license to kill. Thus even though the object a dies at the end of main, the destructor may not get called. Thus, in C# we cannot decide when the destructor gets called. This decision to call the destructor is made by a program within C# called the garbage collector. The concept first gained currency with the advent of Java. In Java and C# we cannot remove our objects from memory. Thus it is for the garbage collector to decide when to call the destructor. The programming world was replete with errors mainly because programmers use new to allocate memory and then forget to deallocate it. This gave rise to a concept called memory leaks. On the flip side of the coin, programmers deallocated the memory, forgot about it and then accessed the object again. This generates an error occurring at random and difficult to pin down. a.cs public class zzz { public static void Main() { aa a = new aa(); } } public class aa { public aa() { System.Console.WriteLine("Constructor"); } public ~aa() { System.Console.WriteLine("Destructor"); } } Compiler Error a.cs(14,9): error CS0106: The modifier 'public' is not valid for this item A destructor cannot have any modifiers like public preceding it. a.cs public class zzz
- 47. { public static voidMain() { aa a = new aa(); } } public class aa { public aa() { System.Console.WriteLine("Constructor"); } ~aa(int i) { System.Console.WriteLine("Destructor"); } } Compiler Error a.cs(14,5): error CS1026: ) expected a.cs(14,10): error CS1002: ; expected a.cs(16,25): error CS1519: Invalid token '(' in class, struct, or interface member declaration a.cs(18,1): error CS1022: Type or namespace definition, or end-of-file expected Constructors come in plenty with different numbers of arguments being passed to them. However, in the case of destructors, one size fits all, i.e., they come in only one size, with no parameters. Here we created a destructor with an int as a parameter thereby confusing the C# compiler completely. C# lacks true synchronous or deterministic destructors i.e. destructors being called at a certain point in time. You cannot have your life depend on when a destructor would be called. The common grouse against C# is that unlike C++ it does not support true destructors. a.cs public class zzz { public static void Main() { aa a = new aa(); } } public class aa { public aa() { System.Console.WriteLine("Constructor"); } ~aa() { System.Console.WriteLine("Destructor"); } protected override void Finalize() { } } Compiler Error a.cs(18,25): error CS0111: Class 'aa' already defines a member called 'Finalize' with the same parameter types We tried to create a function called Finalize. The compiler comes back and tells us that we already have a function called Finalize. This is weird as we have only one Finalize function. The reason for this error is that the compiler converts our destructor from ~aa to Finalize.
- 48. Arrays All programming languagesembrace the concept of arrays. An array is nothing but more of one entity i.e., a multiple of the same type. Simply put, when we have five books, we don't just say we have five books. Instead we say we have an array of five books. So, whenever you want to store multiple values of variables you store them in an array. a.cs class zzz { public static void Main() { int[] a; a= new int[3]; a[0]= 1; a[1]= 10; a[2]= 20; System.Console.WriteLine(a[0]); a[0]++; System.Console.WriteLine(a[0]); int i; i=1; System.Console.WriteLine(a[i]); i=2; System.Console.WriteLine(a[i]); } } Output 1 2 10 20 Here 'a' is an array of ints. You declare arrays with a set of [] brackets. At this point it is not known how large the array will be. For that we have a=new int[3]; after new we have int meaning we want to create a variable of type int. We are putting 3 in the square brackets meaning we want to store 3 ints. This will create three ints a[0], a[1] and a[2]. They are then initialized to the values 1,10 and 20 respectively. To initialize an array variable we use the name, i.e. in this case a and follow it with the open and close [] brackets. Inside them we put the array number. The first variable is called a[0] , the second a[1] and so on. C# like most computer programming languages likes to start counting from 0 and not 1. Therefore, the last variable is a[2] and not a[3]. Since an array is many of the same type, they all have the same name. In our earlier example, even if we have many books, all the books will be called books. However, to refer to them individually you can say book1, book2, book3 etc. WriteLine will display the value stored in a[0] which is 1. a[0]++ will increment the value stored in a[0] by one. WriteLine will now display 2. Thereafter, the variable i is declared as an int. It is then initialized to 1. Within the WriteLine function we have a[i]. It is not specifically stated which variable, instead we have said a[i]. There is no variable called a[i], but i has a value 1, so it is read as a[1]. Hence the value stored at a[1], which is 10, is displayed. The next WriteLine will display the value stored in a[2] as i is reinitialized to 2. Doing this makes our program more generic. We haven't specifically stated which variable, we are letting a variable called i decide the name of the variable.
- 49. The next exampledemonstrates how arrays can be used within loops. a.cs class zzz { public static void Main() { int[] a; a= new int[3]; int i; for( i=0; i<=2; i++) a[i]= i*10; for( i=0; i<=2; i++) System.Console.WriteLine(a[i]); } } Output 0 10 20 The advantage of using arrays is that you can decide the name of the variable later and they can also be used in loops. Here 'a' is an array of ints and i is a variable of type int. The array size is 3 i.e. it can store three ints. The first for loop is used to initialize the individual array items. Within the for loop, i is initialized to 0. The condition i<=2 indicates that the loop will execute thrice. So, when the control enters the for loop, for the first time i is 0. Looking at the right hand side of the expression, i*10 will now read as 0*10, which is 0. Hence, a[i] which is read as a[0] will be initialized to 0. i++ increments the value of i to 1. The second time a[i] will be read as a[1] which will be initialized to 10. Thereafter, a[i] will be read as a[2] and initialized to 20 as i is now 2. Now i will be 3, so the condition evaluates to false and the loop terminates. The second for loop displays the values stored in the array. This is similar to the above loop. Here we are displaying the values of the individual array items. As the for loop executes, WriteLine will read a[i] as a[0], a[1] and a[2] in each case. As such, WriteLine displays 0,10,20. So, starting from the beginning and going upto the end, all the values stored in the array are displayed. Arrays can also be used in a foreach statement. This is exemplified in the following program. a.cs class zzz { public static void Main() { int[] a; a= new int[3]; a[0]= 1; a[1]= 10; a[2]= 20; foreach ( int i in a) System.Console.WriteLine(i); } } Output 1 10 20
- 50. The foreach statementlists the elements of the array. It executes a statement for each element of the array or collection. 'a' is an array of type int that can store three items. a[0], a[1] and a[2] are the elements or items of the array. They have been initialized to 1, 10 and 20 respectively. In the foreach statement we have ( int i in a ). i is a variable of type int and a is the array created earlier. The first element of the array 'a' is a[0] and it holds the value 1. The foreach statement picks up this value and stores it in i. Since i now holds the value 1, WriteLine displays 1. The second element is a[1]. It picks up its value, which is 10 and stores it in i. Thus i now holds the value 10. WriteLine will now display 10. This goes on for all the elements of the array. Our array 'a' comprises three elements. Hence the foreach will execute the WriteLine statement for each of them and display their values 1,10 and 20. The variable i is only available in the foreach. The foreach makes it easier for us to run through all the members of any array. Parameters Revisited with Out and Ref Variables are the cornerstone of programming languages. In a way, they are like items contained in a bag. You can put an item in a bag, remove it therefrom, or replace it with another. Our next few programs shall be explained along these lines. a.cs class zzz { public static void Main() { int i=100; yyy a; a=new yyy(); a.abc(i); System.Console.WriteLine(i); } } class yyy { public void abc(int i) { System.Console.WriteLine(i); i=10; } } Output 100 100 'a' is an object that looks yyy. The variable 'i' is initialized to a value 100. Then the object 'a' that looks like yyy is created. We are now calling the function abc with one parameter 'i'. Within the abc function we are printing the value of i. When we call the function abc from Main, i had a value 100. So the variable i in abc also has a value 100. Thus WriteLine will display the value 100. We are now assigning i a new value 10. But the i that changes is the parameter i in abc, and not the i of Main in zzz. It's like having two similar bags, where you change the contents of only one. So the WriteLine in Main will again print 100. Thus the variable i within the abc function is temporary. It exists only within the '{' and '}' braces of the function. Therefore, this 'i' is local to the function. Its value is not available in Main. It's like your memory, temporary. It exists as long as you are reading this book. Once the book is over everything is forgotten! Thus please do not call the variable i in abc as i because it will confuse everyone. Thus i in abc has nothing to do with the i in Main. Ergo, changing i in abc will have no effect on the i in Main. But a situation could arise wherein you would like the function abc to change the value of i. In this context you are saying, I am giving a variable to a function abc, use it, but I would also like the function abc to change it.
- 51. And once itsvalue is changed in the function, it i.e. the changed value should be available in Main. Let's see how we can handle a situation like that. a.cs class zzz { public static void Main() { yyy a; int i=100; a=new yyy(); a.abc(i); System.Console.WriteLine(i); } } class yyy { public void abc( out int i) { i=10; } } Compiler Error a.cs(8,1): error CS1502: The best overloaded method match for 'yyy.abc(out int)' has some invalid arguments. a.cs(8,7): error CS1503: Argument '1':cannot convert from 'int' to 'out int'. This program is exactly like the previous one. The only change is that we added the word 'out' in the function abc along with the parameter i. We get an error on compiling this program. You realize, we are saying a.abc(i) in Main, but in the function we are saying abc(out int i). 'out' is the key to solving our previous problem. 'out' means whatever changes you will make in abc, they will be visible outside the function also. Note that it doesn't matter what you call the variable in the function abc, the original will change. So, instead of saying abc(out int i) you could have used another variable. In that case, if you said abc(out int p) it would be absolutely legal. To do away with the error you must specify 'out' while calling the function as well. Now we have rewritten the program with the above change. a.cs class zzz { public static void Main() { yyy a; int i=100; a=new yyy(); a.abc(out i); System.Console.WriteLine(i); } } class yyy { public void abc( out int i) { i=10; } } Output 10 In this program, we now have 'out' in both cases: in the function call and the function itself. So by saying a.abc( out i) you are implying that you are allowing the function abc to change the value of i. Then in the function
- 52. definition we alsohave the word 'out', so it knows i is going to change and hence i now changes in the original. Therefore, WriteLine will displays the new value 10. We are now using the word 'ref' instead of 'out'. Let us see the effects of doing so. a.cs class zzz { public static void Main() { yyy a; int i=100; a=new yyy(); a.abc(ref i); System.Console.WriteLine(i); } } class yyy { public void abc( ref int j) { j=10; } } Output 10 This program executes smoothly and gives the same output as previously. It displays the value 10. So in that sense 'ref' is like 'out'. In either case the original value of 'i' changes. Thus, if you are calling the function with 'ref' then state 'ref' in the function also. If you are calling the function with 'out' then give 'out' in the function also. Game for some experimentation? Let's try to execute the above program without initializing the variable i. a.cs class zzz { public static void Main() { yyy a; int i; a=new yyy(); a.abc(ref i); System.Console.WriteLine(i); } } class yyy { public void abc(ref int j) { j=10; } } Compiler Error a.cs(8,11): error CS0165: Use of unassigned local variable 'i' Here we are saying int i. Note that we are not initializing i. We are using 'ref' which gives us an error. The error says 'use of possibly unassigned local variable i'. Here we didn't initialize i and yet we are trying to use it; hence we get an error.
- 53. Now, let's lookat the next example. Here we have the same program only now we are using 'out' instead of ref. a.cs class zzz { public static void Main() { yyy a; int i; a=new yyy(); a.abc(out i); System.Console.WriteLine(i); } } class yyy { public void abc(out int j) { j=10; } } Output 10 In this program we haven't initialized i either but this time we don't get any error. This goes to show that when using 'ref' the variable must be initialized and only then can you change its value. In case of 'out' you don't have to initialize the variable to change its value. But why have this differentiation? This is because in case of 'ref' you can read the value of the variable as well as write to it. Therefore, if one has to read the value of the variable, it must be initialized before hand. But it is your discretion whether you want to change the value, read it or do both. But in case of 'out' you can only write i.e., you can only change the value of the variable, you cannot read it. So, in case of 'out' since it does not read the variable it doesn't matter even if it is not initialized. This is further exemplified with the help of the next few programs. In the following program we have initialized i. Note here we are using 'out'. a.cs class zzz { public static void Main() { yyy a; int i=10; a=new yyy(); a.abc(out i); System.Console.WriteLine(i); } } class yyy { public void abc( out int j) { System.Console.WriteLine(j); j=10; } }
- 54. Compiler Error a.cs(16,26):error CS0165:Use of unassigned local variable 'j' Within the function abc we are first printing the value of i using the WriteLine statement. That means we are trying to read the value of i which is passed to the variable j in the function abc. But with 'out' one cannot read values hence we get an error. Now if you try the same program with 'ref' instead of 'out' you will not get any errors. This is because with 'ref' you can read the values as well as well as change them. However do note that all 'out' parameters must be initialized or else you will get an error form the compiler as follows. a.cs class zzz { public static void Main() { yyy a; int i=10; a=new yyy(); a.abc(out i); System.Console.WriteLine(i); } } class yyy { public void abc( out int j) { } } Compiler Error a.cs(14,13): error CS0177: The out parameter 'j' must be assigned to before control leaves the current method So where can we use 'out'? We can use 'out' if we want the function to make some calculations and fill up the variable. a.cs class zzz { public static void Main() { yyy a; int i=10; a=new yyy(); a.abc( out i ); System.Console.WriteLine(i); } } class yyy { public void abc( out int i ) { i= 20 * 20; } } Output 400
- 55. In this programwe are using 'out'. We are multiplying 20 by 20 and storing the result 400 in i. Thereby the value of i is changed and WriteLine will now print 400. Our next example will make things as clear as daylight. a.cs class zzz { public static void Main() { yyy a; int i=10; int k=20; int m; a=new yyy(); m=a.abc( out i, out k ); System.Console.WriteLine(i); System.Console.WriteLine(k); System.Console.WriteLine(m); } } class yyy { public int abc( out int x, out int y) { int j; x=30;y=10; j=x * y; return j; } } Output 30 10 300 After specifying that a looks like yyy, the two variables i and k are initialized to the values 10 and 20 respectively. Then the object a is created. The third variable m is of type int and is used to store the return value of the function abc. By using 'out', the function abc is called with two parameters, i and k. The abc function has the variables x and y. Within the function another variable j is declared of type int. The variables x and y are assigned new values 30 and 10 respectively. Their values are multiplied and stored in j, i.e. 300. The next statement return j will yield the value of j, which is stored in m. Now WriteLine will display the value of i as 30. This is because with the abc function we filled up i with the value 30. Similarly, the next WriteLine statement will print the value of k, which is 10. Then we print the value of m, which is 300. In a nutshell- In case of both 'ref' and 'out' the original changes. A point to note is that when we say 'ref' it means we will initialize the variable. The abc function expects the variable to be initialized so that the function can use the value of the variable. When the variable changes the change is reflected outside. So, whenever you say 'ref' the variable has to be initialized, otherwise an error occurs. The function will assume that the variable is initialized because it expects to read the value. You may choose to read the value or change it or do both. But when you say 'out' it implies that we are not going to initialize the variable, that the abc function is not going to use the value, it is only going to change the value. When there is an 'out', abc can use the variable even if it is not initialized. This is because you cannot read the value in case of an 'out' variable; the function can only change the value. In other words, it has to change the value of the variable to avoid any errors.
- 56. 4 Components and Database Handling ExceptionHandling Failing to prepare is preparing to fail. Ergo, one is constantly planning and preparing for the future. We often set a path for ourselves and try to follow it steadily. But then life intervenes. Uncertainties in life result in unforeseen situations. These are like exceptions in the normal course that we set for ourselves. Similarly, when you write programs, unforeseen problems may arise during its normal path of execution. These unforeseen problems are nothing but an euphemism for errors. Just as in life, in the programming world, these errors can be further classified into Fatal errors and Non-Fatal errors. A Fatal error is an error that brings the program to a grinding halt. A Non-Fatal error is an error that allows your program to run but with limited capacity. This can be exemplified by the following. Let's assume you have a card that is not of a high resolution. Accordingly, your browser displays your page in a lower resolution. Now, technically, that is an error but it is not a Fatal one. However, if you didn't have a graphics card at all then it would be a Fatal error. Thus, we may also call an unforeseen problem or error an Exception. In other words, therefore, the word Exception is used almost synonymously with the word Error. Earlier, the problem was that we never centralized error handling. Let's assume you have to open three files. Each time you open a file you have to check whether an error occurred or not. So you have to conduct that check for every file. Since there are three files, it would mean repeating the same error check thrice. That is surely a waste of time. Or you could be calling two functions and checking for the same error in both the functions. One reason that programmers don't write error-handling routines is that they get tired of the mundane task. It is the same thing repeated over and over again. Let's consider constructors. Before the constructor gets called, the object has not yet been created. So you ask the constructor to create an object, to allocate memory and create a file. Now, if it can't do so, how will the constructor return to tell you that an error occurred! Today constructors carry a lot of code within them and if you haven't forgotten, constructors cannot return values. Because of the various reasons discussed above, we don't talk about errors any more; we handle exceptions. Bearing this in mind let's understand the next program. a.cs class zzz { public static void Main() { yyy a; a=new yyy(); a.abc(); System.Console.WriteLine("Bye"); } }
- 57. class yyy { public voidabc() { throw new System.Exception(); System.Console.WriteLine("abc"); } } Compiler Warning a.cs(16,1): warning CS0162: Unreachable code detected Output Unhandled Exception: System.Exception: Exception of type System.Exception was thrown at yyy.abc() at zzz.Main() Here, a.abc calls the function abc in class yyy. System.Console.WriteLine is used to display 'Bye'. It is of significance to note that when you run this program the System.Console.WriteLine does not get called. Hence the word 'Bye' is not displayed. Within the abc function we have a line that says throw new System.Exception(); The word new indicates that we are creating an object. We are creating an object that looks like System.Exception. 'throw' is a reserved word, that means it is recognized by C#. Exception is a class in the System namespace. In other words, we are identifying an exception, creating an object of it, and throwing it. Then we have a WriteLine statement for printing 'abc'. Note that neither 'Bye' nor 'abc' gets displayed. A Message Box may appear for debugging the applicaition. Since we are still at the learning stage, we click on the No button. The warning says that when you use the 'throw' keyword in your code, no lines of code get called after that. Since the function abc is throwing an exception no code after the throw in abc will get executed. The throw acts like the return statement. Everything comes to a stand still! And we get an error at runtime and not at the time of running the compiler; indicating where the exception occurred. Also no code gets called after function abc gets called as it throws an exception. Explicitly declaring exceptions tells the compiler that a particular problem might occur. When the problem does occur, an exception is thrown; the next step being to catch the exception. Let's see how we can accomplish this. In our program, the function abc throws an exception. We will now catch the exception. a.cs class zzz { public static void Main() { yyy a; a=new yyy(); try { a.abc(); System.Console.WriteLine("Bye"); } catch (System.Exception e) { System.Console.WriteLine("In Exception"+ e.ToString()); } System.Console.WriteLine("After Exception"); } } class yyy { public void abc() { throw new System.Exception(); System.Console.WriteLine("abc"); }
- 58. } Output In ExceptionSystem.Exception: Exceptionof type System.Exception was thrown. at yyy.abc() at zzz.Main() After Exception Catching exceptions is done within 'try-catch' blocks. Therefore, the code for abc is included within a 'try-catch' block. a.abc - the function that throws the exception - is included within the try-catch block . The abc function throws an exception by using the keyword throw. There is no other way of throwing an exception. At this point all code is skipped in function abc as well as the in the try block and the control moves to the catch block. As such, neither 'abc' nor 'bye' gets displayed. Within the catch we have a parameter 'e' that looks like System.Exception. The object e has a method called ToString. ToString is a very handy function. It tells you where exactly the exception occurred and in which function, function within function. So, System.Console.Writeline will display the string 'In Exception' along with the exception. After the code contained in the catch block is executed, the remaining code after the end of the try - catch block will be executed. Hence, WriteLine will display 'After Exception'. That means the program will not come to a stand still, it resumes execution after the catch and not after the function which threw the exception. If you give a return statement immediately after the catch block, as we have given in the next program, the program will stop execution there itself. Hence, in this case, 'After Exception' will not be displayed as shown below. a.cs class zzz { public static void Main() { yyy a; a=new yyy(); try { a.abc(); System.Console.WriteLine("Bye"); } catch (System.Exception e) { System.Console.WriteLine("In Exception"+ e.ToString()); return; } System.Console.WriteLine("After Exception"); } } class yyy { public void abc() { throw new System.Exception(); System.Console.WriteLine("abc"); } } Output In ExceptionSystem.Exception: Exception of type System.Exception was thrown. at yyy.abc() at zzz.Main() Each time that abc gets called an exception is thrown. But you may not want that to happen. Hence, exception handling is normally included in an if statement and if an error condition takes place.
- 59. A 'try-catch' blockcan include a number of functions and whenever an exception occurs for any one of them, we will catch it. By doing so we are synchronizing all the code to handle errors at one place. Constructors can also throw exceptions. Building Components a.cs public class zzz { public void abc() { System.Console.WriteLine("zzz abc"); } } The above program consists of a simple class zzz with one public function. The class has also been tagged with the modifier public that makes it accessible to everyone. When we run the command csc a.cs, we are asking the C# compiler to create an executable file for us even though we did not explicitly ask it to. An executable file will always be created by the C# compiler unless you override it by stating an option to the C# compiler. Let us start with the /t option. csc /t:library a.cs You could use /t or /target. This option specifies what type of output file the compiler should create for you. If you do not specify a /t option on the command line, C# by default writes /t:exe for you thereby creating an executable file. The long form of /t is /target and depending upon the time of day, choose the appropriate one. Not all options have a long and a short form like /t does. The : after the option is mandatory. Then we write the type of executable output file we want. Library means a dll. When we run the dir command we see a file named a.dll in the current sub directory. A file with a .dll or a .exe extension are called Windows PE files. This is the default file format that Windows uses to create executable files. There is a minor difference between the internal structures of a dll and a exe file. Which is that a dll cannot be executed like an exe program can. By convention dlls' are used to store code even though an exe could have also be used. What we have done is created a component. What if we wanted to change the name of the output file. By default it is the name of our program. Then we must use the /out option as follows. csc /t:library /out:bbb.dll a.cs This will create a file bbb.dll instead of a.dll as earlier. Whether we use the /out option or not, C# does it for us. If the C# program was called a.cs, then C# wrote /out:a.exe on the command line for us. We can use the /out option to change the name of the output file. /out does not have a short form /o. Remember C# uses defaults for command line options to make life easier for us. Note that we have created a component in a file bbb.dll a.cs class yyy { public static void Main() { zzz a; } } Compiler error a.cs(5,1): error CS0246: The type or namespace name 'zzz' could not be found (are you missing a using directive or an assembly reference?) The only reason we create a component is to allow other programs to call code from it. In the above program, we are saying that a looks like a class zzz. The C# compiler is telling us in a vague way that it does not know
- 60. that zzz isa class. Though we know zzz is a class as we just created it and it is in a file called bbb.dll, C# is not aware of the same. a.cs class yyy { public static void Main() { zzz a; } } >csc a.cs /r:bbb.dll Compiler Warning a.cs(5,5): warning CS0168: The variable 'a' is declared but never used The /r or reference option tells the C# compiler to look at bbb.dll; in this case for the code of classes it is not aware of. In our case the error disappears as the file bbb.dll contains the code for the class bbb.dll. Thus in future, if the C# compiler ever gives you the above error, do not panic. All that you need to do is specify which dll contains the code for the classes. The help available along with C#, tells you every class and the dll that contains the code of the class. a.cs class yyy { public static void Main() { zzz a = new zzz(); a.abc(); } } Output zzz abc We have successfully called the function abc in class zzz. The code of the class zzz resides in bbb.dll. a.cs namespace mukhi { public class zzz { public void abc() { System.Console.WriteLine("zzz abc"); } } } >csc /t:library /out:bbb.dll a.cs The same class zzz is now enclosed in a namespace mukhi and the component recreated. a.cs class yyy { public static void Main() { zzz a = new zzz(); a.abc(); } }
- 61. csc a.cs /r:bbb.dll CompilerError a.cs(5,1): error CS0246: The type or namespace name 'zzz' could not be found (are you missing a using directive or an assembly reference?) a.cs(6,1): error CS0246: The type or namespace name 'a' could not be found (are you missing a using directive or an assembly reference?) The error results as the name of the class is not zzz but mukhi.zzz. a.cs using mukhi; class yyy { public static void Main() { zzz a = new zzz(); a.abc(); } } Output zzz abc All's well that ends well. Databases Databases are centralized stores of data. In a database, information from several files (also known as tables) is accessed, coordinated and operated upon as if in a single file. Thus, the database organizes data independently from the programs that access it. Large volumes of data are stored in a database. Computer programs have little meaning when written in isolation. Therefore, it is of importance to have our programs work with databases. Databases work under the control of a database management system. SQL Server is an RDBMS; it is one such database management system. Before we can write programs that communicate with databases we need to have a database. One of the simplest databases to use is Microsoft Access. As a large number of people use it, we have based our examples on it. However you could use any RDBMS like Oracle, SQL Server as you wish. First and foremost, before we can access a database and perform various operations, we need to connect to the database. Assuming you want to speak to your friend over the phone, you dial your friend's phone number. Its only when you connect to your friends phone that the both of you can speak to each other. Similarly, if we want to use a database we first have to connect to it and only then can we speak to it. a.cs class zzz { public static void Main() { System.Data.OleDb.OleDbConnection s; }} Compiler warning a.cs(5,35): warning CS0168: The variable 's' is declared but never used Let's see how we can connect to a database. a.cs class zzz { public static void Main() {
- 62. try { System.Data.OleDb.OleDbConnection s; s =new System.Data.OleDb.OleDbConnection(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output hell Within the try block we have an object s that looks like System.Data.OleDb.OleDbConnection. You are already aware that System is a namespace, but what about Data and OleDb? Well, System.Data.OleDb is the name of the namespace. We can liken this to an earlier example where we created a namespace mukhi.vijay. Hierarchy in a namespace can go to absurd lengths! The help on C# states that System.Data.OleDb is the name of a namespace and OleDbConnection is a class within that namespace. By saying s = System.Data.OleDb.OleDbConnection(); we are creating the object s. When you run this program all that we see is the word hell, which means that the constructor threw no Exception. If it did then the catch block would have been executed. a.cs class zzz { public static void Main() { try { System.Data.OleDb.OleDbConnection s; s = new System.Data.OleDb.OleDbConnection(); s.Open(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output System.InvalidOperationException: The ConnectionString property has not been initialized. at System.Data.OleDb.OleDbConnection.Open() at zzz.Main() The class OleDbConnection has a function called Open which opens or connects to our database. When we run the above the above program, the Open function throws an exception. Now, at this point you must catch the Exception. The ToString within the catch block displays the Exception message. The error says that the ConnectionString property has not been initialized. The reason an exception occurred is that we did not provide certain mandatory information whilst creating the object. We did not indicate the location of the database server that we want to connect to or the database. It's like picking up the phone and not dialing a number. How in the world do you expect to connect then! We will now provide the constructor with some basic information.
- 63. a.cs class zzz { public staticvoid Main() { try { System.Data.OleDb.OleDbConnection s; s = new System.Data.OleDb.OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb;"); s.Open(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output System.Data.OleDb.OleDbException: Could not find file 'c:zzz.mdb'. at System.Data.OleDb.OleDbConnection.ProcessResults(Int32 hr) at System.Data.OleDb.OleDbConnection.InitializeProvider() at System.Data.OleDb.OleDbConnection.Open() at zzz.Main() We first specify the database provider as Provider=Microsoft.Jet.OLEDB.4.0, which stands for MS-Access and then the path as Data Source=c:zzz.mdb separated by semicolon. When you run the program, an OleDbException is thrown, which says could not find file 'c:zzz.mdb'. This is because we have not created an MSAccess DataBase called zzz.mdb. An mdb file stores data from multiple tables. In this file zzz.mdb, create one table abc with 2 fields f1 and f2 , both of type text. Add two records to this database. The first record will contain 1,a1 and the second record will be 2,a2. Re run the program which now displays hell unlike before where we generated an Exception. Output hell s.Open opens a connection to the database. The 'Provider' and 'Data Source' given in the constructor identify the database. As we have given the constructor all the mandatory information, s.Open() will open a connection with the Access database. The program executes smoothly and once the connection is established WriteLine displays 'hell'. Now that we have established a connection with the database, let's see how we can communicate with it. More specifically, let's see how we can execute an SQL command. a.cs using System.Data.OleDb; class zzz { public static void Main() { try {
- 64. OleDbConnection s; s =new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("Create Table a1 ( vno integer , name char(10))",s); c.ExecuteNonQuery(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output hell Employing using, we do not have to preface every class with the namespace System.Data.OleDb. The purpose of this program is to execute an SQL statement. SQL is a short form for Structured Query Language. It is not a computer programming language like C# but rather a language like English. SQL understands certain words like Create, which create a table for us in our database. SQL has nothing to do with Oracle or Access, it works with all the RDBMSs of the world. Create Table a1 (vno integer, name char(10)) The above SQL statement means that a table called a1 will be created in our database. It will have 2 fields vno and name. Vno will store numbers and name will store characters subject to a maximum of 10. A class in any programming language is made up of a collection of variables and functions. These put together do something for us like solving a particular problem or accomplishing a certain task. We live in a world of specialization. Just like it is difficult to come across one human being who is a PHD in Nuclear Science as well as Sociology, classes too are written to focus on one particular task only. Thus we need one class to handle our connection to a database and another class which understands SQL. We first create an object that looks like an OleDbConnection, which understands how to connect to a database using a string attributes 'Provider' and 'Data Source', which it passes as a parameter to the constructor. Then we create an object c which looks like OleDbCommand as this class understands SQL. The constructor gets called with 2 parameters, the SQL statement and also the connection object. Thus the object c now comprises the necessary information regarding the command we want to execute and the connection. Now, c.ExecuteNonQuery() will actually execute the SQL statement. ExecuteNonQuery() is a function within the OleDbCommand class that enables us to execute any SQL command. You will now see the word 'hell' displayed on your screen. But did the table get created? Let's find out by starting Access and then choosing the database zzz. Here we will see the table a1 with 2 fields vno and name but with no data at all. We created our table alright, but it is empty as it has no data in it! So let's start inserting data/records into the table. Before doing so, Quit out of Access a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c;
- 65. c=new OleDbCommand("insert intoa1 values(1,'hi')",s); c.ExecuteNonQuery(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output hell The only difference between this program and the previous one is that this one uses the Insert SQL command. Earlier, we created a table zzz, now we are inserting records in zzz. To the constructor of OleDbCommand we are giving two things - the insert command "insert into a1 values(1,'hi')" and the connection. Insert into and values are part of the SQL syntax. A1 is the name of the table in which a fresh record will be added. After values, we write the data separated by commas. The first field will contain 1 and the second hi. As the second field has the data type of character, it has to be enclosed in single quotes. c.ExecuteNonQuery as usual executes the SQL command and as no exception gets thrown ,the word 'hell' gets displayed.. Let's cross check to see if the data has been inserted into zzz by running Access. Aha! It displays the record that you just inserted in the following format- vno name 1 hi Now it follows that if we can insert a record then we should be able to remove it too. The following program uses the delete command to remove all the records from the table zzz. a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("delete from a1",s); c.ExecuteNonQuery(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output hell Now c.ExecuteNonQuery will execute the delete command. This command deletes all the data from the table. In Access we will see an empty table. This proves that the delete command was successful! Apart from inserting and deleting data another operation that is commonly performed is updating data. However, you must have data in your table to update it. Our table is empty right now, so let's execute the insert program once again. You will now have one record in your table zzz. After inserting a record, change the sql insert command to an update command. This is shown below.
- 66. c= new OleDbCommand("updatea1 set vno=3,name='bad' where vno=1",s); Here, we are giving an update statement and our connection to the object is 'c'. The update statement will change the vno and name to the values specified. It will do so for any record that has vno=1. We have only one record in our table and it meets this criteria. Now, go back to Access , you will find that the record has been updated as follows- vno name 3 bad a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("delete from a11",s); c.ExecuteNonQuery(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output System.Data.OleDb.OleDbException: The Microsoft Jet database engine cannot find the input table or query 'a11'. Make sure it exists and that its name is spelled correctly. at System.Data.OleDb.OleDbCommand.ExecuteCommand TextErrorHandling(Int32 hr) at System.Data.OleDb.OleDbCommand.ExecuteCommand TextForSingleResult(tagDBPARAMS dbParams, Object& executeResult) at System.Data.OleDb.OleDbCommand. ExecuteCommandText(Object& executeResult) at System.Data.OleDb.OleDbCommand. ExecuteCommand(CommandBehavior behavior, Object& executeResult) at System.Data.OleDb.OleDbCommand. ExecuteReaderInternal(CommandBehavior behavior, String method) at System.Data.OleDb.OleDbCommand.ExecuteNonQuery() at zzz.Main() Don't worry, we have simulated this error on purpose. The table a11 does not exist in our database. Thus an exception got thrown by ExecuteCommandText and not ExecuteNonQuery as seen above. What it means is that ExecuteNonQuery calls Execute which calls ExecuteCommand which finally calls ExecuteCommandText. Also the word hell does not get displayed because of the exception thrown. We will now execute a simple select statement. a.cs using System.Data.OleDb; class zzz {
- 67. public static voidMain() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output hell We have executed different SQL commands through our earlier programs. We will now try and execute an SQL select statement. In order to do so, we have given the following select statement to the constructor along with the connection- 'Select * from abc'. Abc is a table in the database called zzz . Remember, c looks like OleDbCommand. 'Select' is part of the SQL syntax like insert , update . 'from' is also a reserved word . 'abc' is the name of the table created earlier in Access and populated by data keyed in by us. The '*' means all fields in the table. We could have also replaced the * with a comma separating list of field names that we were interested in. As OleDbConnection has an Open function, OleDbCommand has a ExecuteReader function. This function returns an object that looks like OleDbDataReader. The function ExecuteReader will create an object that looks like OleDbDataReader and initialize r to it. No exception was thrown, which meant no errors from our side. What we need to do now is retrieve data from the abc table. The abc table may have ten fields and twenty records. So we use what is called a 'dataset'. In other words, how do we access data contained in multiple rows and multiple columns through our program? For example, when we run the select statement it gives us the data in rows and columns. How can we achieve this through our program? To do that we now need a third object which looks like OleDbDataReader. We can almost feel your dismay - Oh no! Another object! Well, there is a very good reason why you need to have three different objects. Firstly, you need an object that understands a connection. Then you need another object that understands an SQL statement. This is because somebody has to execute that statement. You now need a third object that will let you read/retrieve data. That is why we have 'r' an object that looks like OleDbDataReader. Thereafter, we have c.ExecuteReader. Earlier we used a function call ExecuteNonQuery. This is because in case of an insert, update or delete we didn't want any data to be retrieved, we didn't want an answer back!! As a recap, we are using r to store the value return by ExecuteReader. That means r will be filled up by ExecuteReader. On executing this program only 'hell' is displayed. Now that we have the filled up r, let's see how we can display the data. a.cs using System.Data.OleDb; class zzz {
- 68. public static voidMain() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine("{0},{1}",r.GetValue(0),r.GetValue(1)); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output System.InvalidOperationException: No data exists for the row/column. at System.Data.OleDb.OleDbDataReader.DoValueCheck(Int32 ordinal) at System.Data.OleDb.OleDbDataReader.GetValue(Int32 ordinal) at zzz.Main() Run this program. Expect the unexpected....an error! The error says 'No data exists for the row/column' But didn't the function Execute fill up r? It did, but it did not position the record pointer on the first record. A record pointer is an abstract concept. To start with it appears just before the first record. The function GetValue will return the value of the fields depending upon where the record pointer is. At the start it is just before the first record. So r can be also looked at as an array that contains all the fields. We get an error because we need to read the data into our array first. In order to read data into the fields we must give r.Read. Without r.Read, the records will not be read into the array, so also the record pointer will not be moved forward to the first record. Now that we know the reason why the error occurred let's rectify it by doing the needful. Let's add r.Read() to our program. a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine(r.Read()); System.Console.WriteLine("{0},{1}",r.GetValue(0),r.GetValue(1)); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); }
- 69. } } Output True 1,a1 Now that wehave added r.Read() the program executes as advertised. r.Read() returns true. Thereafter, the GetValue() function which needs the field number as a parameter retrieves the data associated with the first and second column. Remember in C# we start counting from zero and hence zero as a parameter to GetValue will retrieve the value of field one in the table. The datatype of the first field is numeric and of the second character. However, the function GetValue does not seem to care. Read returns true if more data is available to read and false if it is at the last record. Also read positions the record pointer to the next record. a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); while (r.Read()) System.Console.WriteLine("{0},{1}",r.GetValue(0),r.GetValue(1)); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output 1,a1 2,a2 (A display of only the first two fields of all the records in the table. ) But now we want to display all the data in the fields. So we are using Read in a loop. while(r.Read()). r.Read() returns true as long as it can read data. The while loop continues till the condition is true. We are using our trusted function WriteLine to display the data of the first and second field. Thus the moment r.Read() cannot read more data it returns false and the while loop terminates. Now all the data in the two fields is displayed and as such we have a long list of data displayed. To verify further, add some more fields and records to abc. Just as we mind our manners in daily life we must do so in programming too. It is a good practice to shut the door when leaving a room. Similarly, in programs we must close what ever we opened. a.cs using System.Data.OleDb; class zzz { public static void Main()
- 70. { try { OleDbConnection s; s =new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); while (r.Read()) System.Console.WriteLine("{0},{1}",r.GetValue(0),r.GetValue(1)); r.Close(); s.Close(); System.Console.WriteLine("hell"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output 1,a1 2,a2 We will mind our manners and give r.Close() and s.Close. Note that it is not mandatory to do so. First we are closing r and then we are closing s i.e. the connection that we opened. a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine(r.FieldCount); for ( int i = 0 ; i < r.FieldCount ; i++) System.Console.WriteLine(r.GetName(i)); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output 2 f1 f2
- 71. An OleDbDataReader hasa useful function called FieldCount which tells you how many fields the SQL statement contained. In our case abc has two fields. We have one more function GetName which will tell us the name of the function in the table given the field number. Thus r.GetName(1) will give us the name of the 2nd field. In the for statement, the variable i starts at zero. The condition becomes false when the value of i is one less than r.FieldCount which in this case is 2 i.e. i will take values of 0 and 1. The reason being i starts from zero as GetName understands zero as the first field. Thus we can now print all the column names using the for. Now lets print the entire table irrespective of the number of columns or the number of rows. a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); while ( r.Read() ) { for ( int i = 0 ; i < r.FieldCount ; i++) System.Console.Write(r.GetValue(i)+" "); System.Console.WriteLine(); } } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output 1 a1 2 a2 You can modify abc to add more fields and fill in more records. Execute the program and see the change in output. Whenever we do not know better, we use a for or a while i.e. a looping construct. However we do not know two things. One, the numbers of rows in the table and two the number of columns. The first while takes care of the number of rows and the for, the variable number of columns. Thus the above program will work for any number of columns and rows in the table. All that we do is change the select statement. The difference between Write and WriteLine is that Write does not add an enter at the end like WriteLine does. Also + is used to join two strings together. We are adding a space at the end of every field as a separator.
- 72. 5 Web Enabling Data Wenow have to our credit, programs that enable execution of a general purpose select statement as well as those that update, insert and delete commands. These were all C# programs which were compiled into executable files. Let's assume you want to display the contents of the customer table. Our program will very well serve this purpose. Alternatively, the program can be made to accept user input and depending upon the table name that the user typed in, the program can display contents of that table. You can easily do so with the help of a function called GetLine in Console. GetLine will cause the program to wait at the console for the user input followed by the 'Enter' key. This situation comes with its share of problems though. The problem is that if there are five people who are going to use this program then you need to give them five copies of the program. If there are five thousand such users then you have to copy this on to five thousand different machines. Simply not possible. Wait a minute! Did you get a sudden brainwave? The perfect solution would be to 'put it on a network'!! But don't forget that there are many networking standards available. Some may have Novell, some may have Windows 2000, and others may have the Macintosh or even Linux/Unix. So what do we do now? You will have to make sure that the same program works under Linux as well as it does under a Mac or any other standard. Apart from this you would have the headache of having to train people to use the program. The only solution to this problem is Web enabling data. This is the new standard of writing code. You need to access your data using Internet Technologies. It's only then that we can eliminate the loopholes that exist within the already available standards. No matter which operating system you install, along with it you get access to a browser. When you use the Internet you use a browser, a.k.a. an user agent, that enables you to view web pages and enables data transfer over the Internet. Since we do have a browser, can't we do a simple thing? Let the user type the name of our machine and get a form. Within the form he can key in the table name and once he does that he will get the details of that table. Which means you will now have to execute your code on a program called the WebServer. This is what is meant by Web Enabling Data. But before we leapfrog into the world of Web Enabling Data, let's acquaint ourselves with the basic language that the browser understands. It is called HTML. HTML Amongst browsers, the usual suspects are Netscape Navigator and Internet Explorer. What happens when you view a web page using such a browser? Technically speaking, you connect to a web server and a file comes over onto your computer, which is generally an HTML file. HTML is the acronym for Hyper Text Markup Language. It is a document-formatting language. HTML is basically a stream of characters without any intelligent functionalities like the for and if statements. Hence it cannot be considered an equivalent of a programming language. HTML, as simple as it is, allows any bumpkin to be an ace at it! Assuming you have a Web Browser, let's write our first HTML program. You can use any editor like word to type your program in. The only thing that you must bear in mind is that the file must be saved with the extension .htm or .html. Create the file in your root directory. In our case, we will save in c: and since we are using the dos editor we shall begin by saying edit a.html
- 73. C:>edit a.html Type thecode as we have done below. a.html Hi Bye Output Hi Bye Note that the text must be on two separate lines. Once you have saved your file run your browser. In the browser you will see a white text box known as the address bar. Click there and type c:/a.html, in our case we have saved the file in the root of the C drive. As of now we are picking up this html file from our hard disk. But when you are on the Net you are accessing a file from some other machine on the net. In that case, you give the address of that machine. For instance, if you want to connect to the Microsoft site, you will type www.microsoft.com. Apart from that there is no difference in trying this on the Net or off the Net. When the browser sees an html file, it reads it line by line. Hence, in the browser window you will see Hi Bye displayed. Wow! You have just written your first html program! But didn't we write Hi and Bye on two separate lines? The browser, dumb as it is doesn't seem to understand this. Looks like the browser, by default, ignores enters! So, how can we make the browser understand? We can do so with the help of 'tags'. HTML is based on 'tags', which instruct the browser how to display a piece of text or an image/picture. A tag begins with '<' and ends with '>'. So let's add a tag. a.html Hi <br> Bye Output Hi Bye Going by the definition of a tag you will realize that br is a tag. It is enclosed within the angular brackets. <br> means next line. Save the file. Now when you view this file in the browser you will find that Hi and Bye are displayed on two separate lines. Hence, HTML is nothing but a collection of tags. You just need to know which tag satisfies what purpose. a.html <b> Hi </b> <br> Bye Output Hi Bye In this program, the tag b means bold. But here we also have </b>, which is a closing tag. This indicates that we are closing the <b> tag. Whatever is enclosed between the two will be made bold. Hence only Hi will be bold. HTML tags are romantic, most of them always travel in pairs - called an opening tag and a closing tag. But some tags like <br> like to play the field for they prefer to remain single. Using the <i> tag we will display Hi in italics. a.html
- 74. <i> Hi </i><br> Bye Output Hi Bye It is all very mechanical! <i> means italics and hence Hi is displayed in italics. Had you included bye within <i> and </i> then bye would be in italics too! That's all the understanding that is required to learn HTML! Now we have included another tag <h1> a.html <i> Hi </i> <br> <b> Bye </b> <br> <h1> You are Welcome again! <h1> Output Hi Bye You are Welcome again! Since Hi is included in <i> </i> it will be displayed in italics. Bye is displayed in bold due to the <b> tag. <hi> means heading1, it makes the text bigger. Thus, 'You are Welcome again!' is displayed in a bigger font. a.html <html> <body> <i> Hi </i> <br> <b> Bye </b> <br> <h1> You are Welcome again! <h1> </body> </html> Output Hi Bye You are Welcome again! This program outputs the same result as the above. The only difference is that now it is a well-formed HTML program. Every HTML file must begin with the <html> and end with </html> tag. Whatever text is to be displayed within the browser must be enclosed within the <body> and </body> tag. This is the way an HTML program must be written. Hope the purists have forgiven us now! Let's make our page attractive by adding a picture to it. Copy any gif file to your root directory and name it aa.gif. In our case, we copied it to c: a.html <html> <body> <i> Hi </i> <br> <b> Bye </b> <br> <img src="aa.gif"> <h1> You are Welcome again! <h1> </body> </html>
- 75. img is theimage tag, it is used to display pictures. Along with this tag you have to specify the name of the image file. Following the word 'img' is the word 'src' and after the '=' sign you specify your filename. i.e. aa.gif. You can give the name of any picture file. But follow this syntax! Another thing to note is that you can have the file name included in single quotes or double quotes or you may exclude them totally. Thus viewing this file in the browser will now display the image you specified along with the text. In HTML parlance, src is called an attribute. An attribute describes / qualifies a tag. <a href=a.html> Hi </a> <a href=b.html> Bye </a> You will see two hyperlinks in your browser window. The names of the html files will not be displayed but the words Hi and Bye will be underlined. And if you click on hi and bye the respective html files will be displayed instead. Then there are tables in HTML a.html <html> <table border=10> <tr> <td>hi</td><td>100</td> </tr> <tr> <td>1000</td><td>bye</td> </tr> </table> </html> The table tag has one attribute, border, that specifies how the lines or borders between columns looks like. The table tag encompasses two tags . tr stands for a table row and td for a table columns. We have two tr's , hence we have two rows and each tr encloses two td's one for each column. Similarly, you can keep adding more and more tags depending on how you want your page to be displayed. Any book on HTML will list all the available tags. Our aim is not to learn html here but to use C# on the Net. Since knowing this much will suffice our forthcoming needs, let's get back on track! Using C# on the Net Now that you are familiar with the rudimentary elements of HTML, let's see how we can apply our recently acquired knowledge in conjunction with C# on the Net. First and foremost you need a Web Server. Hence you will now need to install Apache. You can download Apache from http://www.apache.org and then run the installer program. Among the Apache directories, there are two sub-directories that are of importance to us, namely, cgi-bin and htdocs. The cgi-bin sub-directory is used for storing executable files. In order to change to the cgi-bin sub-directory follow the path as given below. C:progra~1apache~1apachecgi-bin And to change to the htdocs sub-directory give the following path. C:progra~1apache~1apachehtdocs The htdocs sub-directory is used for storing html files. To install the Apache Web Server all that you need to do is download the install program on your hard disk. Run the executable program which will install apache in the apache group directory of program files. Change to C:progra~1apache~1apache directory and then run the web server as follows.
- 76. C:progra~1apache~1apache >apache If youget an error about some server name, cd to the conf sub directory , edit a file called httpd.conf , change a line starting with #ServerName to ServerName localhost and all should be well after that. Now that you have the Apache server running, activate your web browser. Type the address 127.0.0.1 in the address bar and press enter. Every machine on the internet is known by a number. We call this number an IP address. IP stands for the Internet Protocol. Every machine that has the Internet software running is also known by the number 127.0.0.1 or localhost. In case a file called index.html exists, the browser will display it. This is because it is located in htdocs and is the file that gets picked up by default when you give your machine address and you have Apache running. Change to the htdocs sub-directory and create an html file a.html to display a form. a.html <html> <form> <input type=submit value=Search> <form> </html> The <form> tag is used to specify that we are creating a form. <input type=submit is the syntax for creating a button. Value =Search means that 'Search' is the word that will appear on the button. Had we said Value = Vijay, you would see the 'vijay' on the button. Save the file as a.html in the htdocs subdirectory. Go to the browser window and type http://127.0.0.1/a.html in the address bar and press enter. You will now see a button with the name Search on it. But clicking on this button doesn't serve any purpose as of now. Let's alter the program such that when the user clicks on the Search button our C# program should execute. So first we will write our C# program. C:csharp>edit a.cs Now type the following code. a.cs class zzz { public static void Main() { System.Console.WriteLine("hi<b>bye"); } } Output hi<b>bye This is a simple program that should display hi and bye. Save the program as a.cs. On compiling the program an executable file 'a' is created. If you run the program as we have been doing, you will see the output as given above. But remember, what we just told you about executables? When you are using Apache you must save all executables in the cgi-bin sub-directory as a security precaution. Hence you must copy a.exe to cgi-bin by giving the following command. C:csharp>copy a.exe c:progra~1apache~1apachecgi-bin
- 77. Now we willalter our HTML program. Within the form tag include action=http://127.0.0.1/cgi-bin/a.exe This indicates the action to be performed when the button is clicked. Since we want our program a.exe to run, we have given its path. a.html <html> <form action=http://127.0.0.1/cgi-bin/a.exe> <input type=submit value=Search> <form> </html> Now that we have everything ready, let's get on with the show! Activate your browser and type http://127.0.0.1/a.html in the address bar. You see the button labeled Search. Now click on this button. To your horror you see an error! The browser window displays an Internal Server Error. Note the url in the address bar- http://127.0.0.1/cgi-bin/a.exe? Now make the changes as we have done below. a.cs class zzz { public static void Main() { System.Console.WriteLine("Content-Type:text/htmln"); System.Console.WriteLine("hi<b>bye"); } } Output in Browser hibye Save your file, compile it and again copy it to cgi-bin once again. Now when you run your program from the browser, the error vanishes! And you will see hibye displayed. That means it is mandatory to give Content- Type. It tells the browser that what is being sent across is a text file. Also the content is qualified as html and not pure text. It is also called a header. Because we are saying html, the bye is displayed in bold. So we created a file and the file goes over i.e. what ever is in Console.WriteLine goes over. Now remove html and instead write plain. Let's see what happens. After making the necessary changes and compiling the program copy it again to cgi-bin. a.cs class zzz { public static void Main() { System.Console.WriteLine("Content-Type:text/plainn"); System.Console.WriteLine("hi<b>bye"); } } Output in Browser hi<b>bye Now run your program from the browser. You will notice that bye is not displayed in bold. Also note that now the tags are displayed as normal text. They show no formatting effects. This is because instead of the word html we are now saying plain. That means the browser will now display everything as plain text. Thus the Content-Type: header is used to indicate to the browser that we are sending text followed by HTML. Thus if we want our program to execute on the web it is mandatory that we include this header in our program as the web server does not know what is the file type it is sending over.
- 78. Now that wehave all the ingredients for the final show, let's get on with it. We will now add the header to our general-purpose select program. We will do this so that we can display the contents of a table on the web. Thus the only change that we have made to the program is to add the following line: System.Console.WriteLine("Content-Type:text/htmln"); a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand("select * from abc",s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine("Content-Type:text/htmln"); while ( r.Read() ) { for ( int i = 0 ; i < r.FieldCount ; i++) System.Console.Write(r.GetValue(i)+" "); System.Console.WriteLine(“<br>”); } } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } We have also added System.Console.WriteLine("<br>"); so that each record is displayed on a new line. The browser will see <br> and understand it as an html tag. This is because in the header we have specified text/html. As such you will see each record on a new line. Now save the file and compile it. Copy the file a.exe to the cgi-bin sub-directory. Activate the browser window and type http://127.0.0.1/a.html in the address bar. Now click on the search button. Accolades! You have successfully run your C# program from the browser. You will now see all the contents of the abc table. Why the abc table? Because that's the table you specified in your program. Thus, whenever the user clicks on the button in the browser he is calling a program on the server. The server could be a trillion miles away!! However, even now everything is not hunky dory. The major problem being that this entire approach is inflexible. Earlier, we told you we need this type of programming because we didn't want to copy the programs on each machine. But the problem with the Web Server is that if it contains ten html files then we can give the user only those ten html files. So we are only giving him ten views. However, what we really need to do is that we need to pass data from us to his machine. For example, let's assume we have a database that has names of people. We should have a form in which the user can write the name of a person. When he clicks on 'ok' that name should get added to the database. In effect, we are adding one person’s data to the database. Similarly, we should be able to delete a person from the database too. And finally we can have a table-display program wherein the user will provide the table name. When he clicks on ok, the contents of that table will be displayed. Thus you can now think of generating
- 79. flexible programs. Youprobably felt limited earlier as you could display contents of only that table whose name you had provided within the program. Environmental Variables Before we get down to making our programs flexible we need to understand as to how we can deal with Environmental Variables in C#. An Environmental Variable is a word that the operating system understands. a.cs class zzz { public static void Main(){ string s; System.Console.WriteLine("Content-Type:text/htmln"); s=System.Environment.GetEnvironmentVariable("PATH"); System.Console.WriteLine(s); } } Output Content-Type:text/html C:JDK1.2.2BIN; C:WINDOWSMICROSOFT.NETFRAMEWORKV1.0.2204; C:WINDOWS; C:WINDOWSCOMMAND; C:BORLANDCBIN; C:WINDOWS; C:WINDOWSCOMMAND; C:PROGRAM FILESMTS Displays the value of environmental variable called PATH. An environmental variable is a word, which stores a value. When you give the command set xyz=bye at the command prompt, xyz becomes an environmental variable, which stores the value bye. All operating systems allow you to create environmental variables . The environmental variable PATH stores a list of directories that the operating system searches for to find executables files. Run it off the web server or by itself. a.cs class zzz { public static void Main(){ System.Collections.IDictionary i; i=System.Environment.GetEnvironmentVariables (); System.Collections.IDictionaryEnumerator d; d=i.GetEnumerator(); System.Console.WriteLine("Content-Type:text/htmln"); System.Console.WriteLine(i.Count + "<br>"); while (d.MoveNext()) { System.Console.WriteLine("{0}={1}<br>",d.Key,d.Value); } } } There can be 100's of environmental variables and we would like a list of all of them. The class Environment in the System namespace has a function GetEnvironmentVariables which returns an object that looks like IDictionary. This IDictionary object has a function called GetEnumerator which returns an object that looks like IDictionaryEnumerator. You have to learn all this by rote, there is no other option available. We are then displaying how many environmental variables there are by printing a member Count in IDictionary. Then we need to activate each variable by calling MoveNext which returns true if there are more environmental variables in the list. An environmental variable is represented by a key-value pair, which is also variable in the IDictionaryEnumerator class. Thus we can display all the environmental variables starting with a count of how many there are. If you check the list of environmental variables in your browser, you will see one called QUERY_STRING , but it will have no value.
- 80. Now run ashttp://127.0.0.1/cgi-bin/a.exe?aa=ho and see the variable QUERY_STRING have a value aa=ho. a.html <html> <form action=http://127.0.0.1/cgi-bin/a.exe> <input type=submit value=Search> <input type=text name=aa> <form> </html> Run the file in the browser. And you will now see a text box which is internally called aa. This is because of the tag input type=text. Type ho in the text box and click on search. The URL will now change to http://127.0.0.1/cgi-bin/a.exe?aa=ho and the variable QUERY_STRING will have the value aa=ho. a.html <html> <form action= http://127.0.0.1/cgi-bin/a.exe > <input type=submit value=Search> <input type=text name=aa> <input type=text name=bb> <form> </html> Now we will see two text boxes in which we will type in ho and no. Click on Search and the url will change to http://127.0.0.1/cgi-bin/a.exe?aa=ho&bb=no and the variable QUERY_STRING will have the value aa=ho&bb=no. CGI Programming Let us now understand what goes on once again. What we have been trying to explain to you is called CGI programming where CGI stands for Common Gateway Interface. Everyone refers to by its acronym rather than its expanded full form. CGI is a means by which a client i.e. a browser can send some information to the web server. Now typically what happens when we land up at a search engine? Well, we want to send the web server i.e. yahoo the words that we want it to search the internet for. At times we are asked to fill up a form on the web asking for some personal details. The information we key in is sent across to the web server. Lets start with the above HTML file that makes CGI happen. We have two input tags which have an attribute type=text. Thus we see two text boxes on our screen. We will type in them hi and bye respectively. The third input tag has a type=submit and another attribute value=search. This makes it into a button with the word search written on it. Now, when we click on search after typing in data in the text boxes, the browser knows that you have clicked on a button of type submit. So, it will now look for a tag form , and when it finds it , it will read the attribute action. Whatever is the value of action, it will write it in the address bar. In the address bar we write a URL or Uniform Resource Locator or the address of a computer on the net. In our specific case, our browser's address bar will contain http://127.0.0.1/cgi-bin/a.exe. The browser does not stop here. It then adds a ? to the end of the above URL . Now it looks at every text box that we have in the form. The first one has been given a name aa and we type hi in it. So our URL now changes to http://127.0.0.1/cgi- bin/a.exe?aa=hi. It's not over yet as we have one more text box to finish with. This one is called bb and has bye in it. Thus our URL now reads http://127.0.0.1/cgi-bin/a.exe?aa=hi&bb=bye. The & is used to separate different parameters. aa and bb are also called parameters. This url is sent across to the web server. The web server is just another name for a file server. It can only send files across. In this case we are asking it to run a program called a.exe from the cgi-bin sub directory. Before the web server runs a.exe, it creates one environmental variable called QUERY_STRING and initializes it to whatever follows the ? i.e. in this case aa=hi&bb=bye. The web server now runs a.exe. What a.exe actually does is none of its concern. All that is wants from a.exe is a html file which the web server sends back to the browser.
- 81. The web serveralso needs at least one header Content-Type which tells it the type of file being sent across. Normally a.exe would read the environmental variable QUERY_STRING, extract the parameters, then read a database, extract some values from it and finally create an HTML file which it would hand over to the web server. All in all, this is how the magic of the internet is bought about. Now, let's learn some more C#. a.cs class zzz { public static void Main(){ string s ; string [] e; char[] c ; s = "hi=bye&no=bad" ; c = new char[2]; c[0] = '&'; c[1] = '='; e = s.Split(c); foreach ( string t in e) System.Console.WriteLine(t); } } Output hi bye no bad s is a string which has been initialized to hi=bye&no=bad which looks similar to what QUERY_STRING as explained earlier looks like. C is an array of chars which has two members initialized to & and = respectively. The string class has a member Split, which will split a string as per the delimiters provided as the first parameters in an array of chars. Thus the split function will take the string s and split it whenever it sees a & or a = . In this case it will result in 4 strings. Split returns an array of strings which we are storing in an array e and using foreach to display the array. Now lets us create a simple form based web application which will accept a table name and then display the entire table for us. The html file a.html as usual will be copied to the htdocs sub-directory. a.html <html> <form action= http://127.0.0.1/cgi-bin/a.exe > <b> Enter Table Name </b> <input type=text name=aa> <p> <input type=submit value="Show Table"> <form> </html> We will run as: http://127.0.0.1/a.html Our C# program freshly compiled and copied to the cgi-bin subdirectory would now read as: a.cs using System.Data.OleDb; class zzz {
- 82. public static voidMain() { try { string t; t=System.Environment.GetEnvironmentVariable("QUERY_STRING"); string [] u; char [] v = new char[1]; v[0] = '='; u = t.Split(v); string w; w = "select * from " + u[1]; OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand(w,s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine("Content-Type:text/htmln"); while ( r.Read() ) { for ( int i = 0 ; i < r.FieldCount ; i++) System.Console.Write(r.GetValue(i)+" "); System.Console.WriteLine("<br>"); } } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } At our web form we are asked to write the name of the table. Then click on the button Show Table. The web browser now writes the following URL in the address bar after we wrote abc as the name of our table. http://127.0.0.1/cgi-bin/a.exe?aa=abc. The Apache Web Server now runs a.exe. a.exe is nearly similar to what we wrote earlier with some minor modifications. t contains the value of the environmental variable QUERY_STRING . v is an array of chars with one member which is our delimiter '='. Split will return an array of strings in u , in our case aa and abc which will be stored in u[0] and u[1]. Earlier we hard-coded the SQL Select statement. Here w starts with the constant string 'Select * from ' and the name of the table is got from u[1] which in turn gets its value from the environmental variable QUERY_STRING. Write the name of any table that exists in the database in the text box and see how you have been able to web enable your data on the net. Lets use the table tags to make our data look more appealing in the browser. a.cs using System.Data.OleDb; class zzz { public static void Main() { try { string t; t=System.Environment.GetEnvironmentVariable("QUERY_STRING"); string [] u; char [] v = new char[1]; v[0] = '='; u = t.Split(v);
- 83. string w; w ="select * from " + u[1]; OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand(w,s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine("Content-Type:text/htmln"); System.Console.WriteLine("<table border=10>"); while ( r.Read() ) { System.Console.WriteLine("<tr>"); for ( int i = 0 ; i < r.FieldCount ; i++) { System.Console.Write("<td>" + r.GetValue(i) + "</td>"); } System.Console.WriteLine("</tr>"); } System.Console.WriteLine("</table>"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } The output looks great. We have made very small additions to our program, mostly cosmetic. We start with the table tag with a border. The while gets called once for every record and in this while we start and end with a row tag tr and /tr. The for gets called for every field and here we enclose the field value with a tag td and /td. At the end of the while we end with the /table tag. The only problem is that we have not displayed the field names at all. Let the next program do so. a.cs using System.Data.OleDb; class zzz { public static void Main() { try { string t; t=System.Environment.GetEnvironmentVariable("QUERY_STRING"); string [] u; char [] v = new char[1]; v[0] = '='; u = t.Split(v); string w; w = "select * from " + u[1]; OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand(w,s); OleDbDataReader r; r = c.ExecuteReader(); System.Console.WriteLine("Content-Type:text/htmln"); System.Console.WriteLine("<table border=10>"); System.Console.WriteLine("<tr>"); for ( int i = 0 ; i < r.FieldCount ; i++)
- 84. System.Console.WriteLine("<td>" + r.GetName(i)+ "</td>"); System.Console.WriteLine("</tr>"); while ( r.Read() ) { System.Console.WriteLine("<tr>"); for ( int i = 0 ; i < r.FieldCount ; i++) { System.Console.Write("<td>" + r.GetValue(i) + "</td>"); } System.Console.WriteLine("</tr>"); } System.Console.WriteLine("</table>"); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } All that we did was to copy the for loop which generated the field names from an earlier program. We put a tr and a /tr around the for loop and the names of fields are tagged with td. Great looking output. a.cs class zzz { public static void Main() { string t; t=System.Environment.GetEnvironmentVariable("QUERY_STRING"); System.Console.WriteLine("Content-Type:text/htmln"); System.Console.WriteLine(t); } } Output aa=select+*+from+abc In the original a.html instead of writing a simple table name we wrote ‘select * from abc’ instead. When we displayed the environment variable QUERY_STRING we learn that the spaces are replaced with a + sign. The rules of creating a URL specify that a space is a forbidden character and all spaces that we write are replaced by a +. Thus we have to convert the + back into spaces. a.cs class zzz { public static void Main() { string t,s; t=System.Environment.GetEnvironmentVariable("QUERY_STRING"); System.Console.WriteLine("Content-Type:text/htmln"); s = t.Replace('+',' '); System.Console.WriteLine(s + "<br>"); System.Console.WriteLine(t); } } Output aa=select * from abc aa=select+*+from+abc The string class has a method called replace which replaces every occurrence of the first parameter i.e. + with the second i.e. a space. Thus s is t but with the plus sign replaced by a space.
- 85. Lets us knowwrite an insert statement which will ask the user to key in his first and last name and then add it into a table. We create a table 'bbb' in our database with two fields f1 and f2 both character type. Our html file also now changes to. a.html <html> <form action= http://127.0.0.1/cgi-bin/a.exe > <b> Enter Users First Name </b> <input type=text name=aa> <p> <b> Enter Users Last Name </b> <input type=text name=bb> <p> <input type=submit value="Add"> <form> </html> a.cs using System.Data.OleDb; class zzz { public static void Main() { try { OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; string t; t=System.Environment.GetEnvironmentVariable("QUERY_STRING"); string [] u; char [] v = new char[2]; v[0] = '='; v[1] = '&'; u = t.Split(v); string f1,f2,f3; f1 = u[1].Replace('+',' '); f2 = u[3].Replace('+',' '); f3 = "insert into bbb values('" + f1 + "','" + f2 + "')"; System.Console.WriteLine("Content-Type:text/htmln"); System.Console.WriteLine(f3); c=new OleDbCommand(f3,s); c.ExecuteNonQuery(); } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Output insert into bbb values('vijay ram','mukhi') URL in Address Bar. http://127.0.0.1/cgi-bin/a.exe?aa=vijay+ram&bb=mukhi The first change is that our array of chars is now two members large and the second delimiter is the '&'. Thus our QUERY_STRING will break into 4 strings aa , vijay+ram , bb and mukhi in u[0] to u[3]. We replace the + with the ' ' in both strings and dynamically generate the insert statement. It looks confusing but is not, Once again f1 and f2 contain the dynamic data but because they are strings we need them to be separated by a single
- 86. inverted comma. Thusthe complication in concatenating strings. The browser displays the SQL statement that will be executed, but in real life we would display a success or error message. Data Search Program Now let us write a search engine or more precisely, a general purpose data retrieval engine. For which we have to first create a simple database that will contain the following fields : Keyword - that will store what the page contains, url - the address of the page, name - to be displayed instead of the url and finally the text that will be displayed explaining the page. create table ss ( keyword char(10), url char(100), name char(100), descrip char(100)); We will also add the following records. insert into ss values ( 'java', 'a.html', 'java site', 'great java site'); insert into ss values ( 'c', 'b.html', 'c site', 'great c site'); insert into ss values ( 'basic', 'c.html', 'basic site', 'great basic site'); insert into ss values ( 'cobol', 'd.html', 'cobol site', 'great cobol site'); Create the following html files in the htdocs subdirectory search.html <html> <form action= http://127.0.0.1/cgi-bin/a.exe > <b> Enter keyword </b> <input type=text name=aa> <p> <input type=submit value="Search"> <form> </html> a.html The file is for java b.html The file is for C c.html The file is for Basic d.html The file is for Cobol The C# program will be as follows a.cs using System.Data.OleDb; class zzz { public static void Main() { try { string t; t=System.Environment.GetEnvironmentVariable("QUERY_STRING"); string [] u; char [] v = new char[1]; v[0] = '='; u = t.Split(v); string w;
- 87. System.Console.WriteLine("Content-Type:text/htmln"); w = "select* from ss where keyword='" + u[1] + "'"; OleDbConnection s; s = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:zzz.mdb"); s.Open(); OleDbCommand c; c=new OleDbCommand(w,s); OleDbDataReader r; r = c.ExecuteReader(); while ( r.Read() ) { System.Console.WriteLine("<a href=/{0}> {1} </a> {2} <br>",r.GetValue(1),r.GetValue(2),r.GetValue(3)); } } catch(System.Exception e) { System.Console.WriteLine(e.ToString()); } } } Open up the browser and enter the url 'http://127.0.0.1/search.html' All that this program does is use the concept of a hyperlink to build a search engine and extract data from a database. We assume we have a database of a trillion records that map the internet telling us the keywords that each url stands for. The user keys in the name of a language and we use the select statement to fetch the records meeting the condition, format it and then send the html file across. We can use the same concept to fill up a list box. The html tags for list box are as follows <select name = aa> <option> blue </option> <option> red</option> <option> orange </option> </select> In a real life situation, the colours will be retrieved from a database and the HTML file generated by a CGI program. 6 Miscellaneous Multiple files A C# program can be spread over multiple files. So far all our code has been written in one large file. Lets us create 2 .cs files a.cs and b.cs as follows a.cs class zzz {
- 88. public static voidMain() { yyy a = new yyy(); a.abc(); } } b.cs public class yyy { public void abc() { System.Console.WriteLine("hi"); } } Earlier our code did not spawn multiple files. C# does not care whether the code is in one file or spread over multiple files. We have only to make a small change while we compile the program. Running Csc a.cs b.cs will create a.exe Output Hi These files are called source files and it is a good idea to give them a file extension of .cs. If you rename b.cs to b.xxx as we did and rerun csc as >csc a.cs b.xxx This will create a.exe as usual. File extensions matter to the programmer not to the compiler. Ascii and Unicode a.cs class zzz { public static void Main() { System.Console.WriteLine((char)65); System.Console.WriteLine((char)66); System.Console.WriteLine((char)67); } } Output A B C Computers in a way are pretty dumb. They do not understand letters of the alphabet. All that they can store in memory are numbers. But then, how does a computer understand or display alphabets? The WriteLine function displays 65 as 65, but the output is A. In the () brackets we have placed a data type called char. We call a ( ) a cast. It means, for the moment convert whatever follows into a char. Thus the number 65 gets converted into a char which is displayed as a 'A'. The 66 gets displayed as a 'B'. Someone, somewhere in the world invented a rule which specified that the number 65 represents a capital A, etc. This rule is given a name and is called ASCII. All that ASCII says is that the numbers form 0 to 255 can also represent small and capital letters, punctuation etc. Whenever you write A, rest assured somewhere in memory, a 65 was stored. A file on disk can also contain numbers form 0 to 255 only and the same rule as spelt above apply.
- 89. a.cs class zzz { public staticvoid Main() { char i = 'a'; System.Console.WriteLine((char)i); } } Output a C# offers us the data type char to represent ASCII values naturally. a.cs class zzz { public static void Main() { int i; for ( i=0; i<=255; i++) System.Console.Write(i + " " + (char)i + " "); } } The above program displays the entire Ascii table. The problem with Ascii is that it is sufficient for a language like English, which does not have to many symbols to represent. However, when it comes to visual languages like Japanese, they have more symbols to represent than English. Ascii can represent a max of 256 unique symbols. The industry thus invented Unicode which uses 2 bytes for every character unlike Ascii's one. All the languages of the world can be represented by Unicode. C# understands Unicode and thus the char data type store characters internally as Unicode and not Ascii. The present Unicode standard is 3.0. a.cs class zzz { public static void Main() { int int; } } Compiler Error a.cs(5,5): error CS1041: Identifier expected, 'int' is a keyword a.cs(5,8): error CS1001: Identifier expected Words like int, char, if etc are reserved by C# and we are not allowed to use them as function names, class names, variable names etc. However, if you insist on doing so, then you have to preface the name with a @ sign like below. a.cs class zzz { public static void Main() { int @int; }
- 90. } Compiler Warning a.cs(5,5): warningCS0168: The variable 'int' is declared but never used The warning does not name the variable @int but int. a.cs class zzz { public static void Main() { int @int; @int = 10; System.Console.WriteLine(@int); } } Output 10 We have millions of names to choose for a variable, then why insist on an int. There will be times when another language declares a name as a reserved name and in those cases we would use the @ sign. It is advisable not to use the @ sign very often. When we run the C# compiler on our program, it does 2 things. One, it reads our code and converts it into things/tokens it understands. This is called a Lexical analysis. Then it does a Syntactic analysis which gives us an executable output. Comments Comments are a form of documentation. They are lines of code written for our benefit (the community of programmers) and not for C#'s. In spite of this, programmers in general are lazy in writing comments. Comments are ignored by the compiler. a.cs // hi this is comment class zzz { public static void Main() // allowed here { /* A comment over two lines */ } } A regular comment starts with a /* and ends with a */. They can be spread over multiple lines and can be placed anywhere in your code. Any line beginning with a // is a one line comment and as the name suggests, cannot span multiple lines. A single line comment does not have to be at the beginning of a line. Escape Sequences and Strings a.cs class zzz { public static void Main() { System.Console.WriteLine("hi nByetNo"); System.Console.WriteLine("");
- 91. } } Output hi Bye No An escapesequence is anything that starts with a . A n means start printing from a new line and a t means a tab. Two backslashes convert into a single backslash. a.cs class zzz { public static void Main() { System.Console.WriteLine(@"hi nByetNo"); } } Output hi nByetNo A string is anything in double quotes. A verbatim string starts with a @ sign and all the escape sequences are ignored by the C# compiler and displayed verbatim. a.cs class zzz { public static void Main() { System.Console.WriteLine("hi bye"); } } Compiler Error a.cs(5,26): error CS1010: Newline in constant a.cs(6,6): error CS1010: Newline in constant A string cannot spawn multiple lines. a.cs class zzz { public static void Main() { System.Console.WriteLine(@"hi bye"); } } Output hi bye Placing an @ in front of the string lets it spawn multiple lines and the spaces shown in the output. If you want the to lose its special meaning in a string, preface that string with a @ sign. a.cs class zzz { public static void Main() {
- 92. string a ="bye"; string b = "bye"; System.Console.WriteLine(a == b); } } Output True The above example displays true, even though the two strings may be stored in different areas of memory. The two strings contain the same characters and hence are similar. The Preprocessor Before the C# compiler can start, a small part of it called the pre-processor first activates itself. It is called the preprocessor as the same concept existed in the programming language 'C'. All that the preprocessor does is that it looks at those lines beginning with a # symbol. a.cs #define vijay class zzz { public static void Main() { } } The first preprocessor directive we are learning is called define. This lets us create a word/variable or even better, an identifier called vijay. The identifier vijay has no value other than true. a.cs class zzz { public static void Main() { #define vijay } } Compiler Error a.cs(5,2): error CS1032: Cannot define/undefine preprocessor symbols after first token in file We cannot use the #define, after valid C# code has been written. They have to come at the beginning of the file. a.cs #define vijay class zzz { public static void Main() { #if vijay System.Console.WriteLine("1"); #endif } } Output 1
- 93. As a #definecreates a variable, its value can be checked by the if or more precisely the #if which works in the same way as the if of C# does. Thus the #if is true and all code up to the #endif gets added to the code. a.cs class zzz { public static void Main() { #if vijay System.Console.WriteLine("1"); #else System.Console.WriteLine("2"); #endif } } Output 2 The same rules as before for an else. Here as we have not created an identifier called vijay, it gets a value of false and therefore the #if is false. Imagine a preprocessor identifier as a boolean variable. Why use a preprocessor variable instead of a normal one? Run the C# compiler as follows on the above program and observe the change in output. csc /D:vijay a.cs Output 1 The output displays 1 as the /D compiler option lets you create identifiers at the time of compiling the program. This cannot be done with a normal variable. We can add/subtract lots of code form our program, at the time of compilation. a.cs #undef vijay class zzz { public static void Main() { #if vijay System.Console.WriteLine("1"); #else System.Console.WriteLine("2"); #endif } } Output 2 As we are allowed to create an identifier vijay by the define, the undef does the reverse. It sets it to false which is the default in any case. As the value of vijay is false, the else gets activated. However we run the above as csc /D:vijay a.cs, we are first creating an identifier vijay at the command line prompt, then undefining it at the first line in the program and the output will show 2 as before. You cannot use the define or undefine after real code. a.cs #define vijay #undef vijay #undef vijay class zzz
- 94. { public static voidMain() { #if vijay System.Console.WriteLine("1"); #endif } } People are allowed to nag you as many times as they like. Repetition has been part of human history since ancient times. You are allowed to undef as many times as you like even though it makes no logical sense. a.cs #define vijay #define mukhi class zzz { public static void Main() { #if vijay #if mukhi System.Console.WriteLine("1"); #endif #endif } } Output 1 You can have as many #if's within #if's. We call them nested if's. If the #if is true, then the text up to the #endif is included. a.cs #define vijay class zzz { public static void Main() { #if vijay System.Console.WriteLine("1"); #else int int; #endif } } We get no error at all in spite of the fact that we are not allowed to create a variable called int. Is C# sleeping at the wheel? It is not as the preprocessor realized that the identifier vijay is true, it removed all the code from the #else to the #endif. C# did not flag an error at all, as it was not allowed to see the offending code by the preprocessor. a.cs class zzz { public static void Main() { #if vijay System.Console.WriteLine("1"); #else int int;
- 95. #endif } } Compiler Error a.cs(8,5): errorCS1041: Identifier expected, 'int' is a keyword a.cs(8,8): error CS1001: Identifier expected Now we see the error as the identifier vijay is false. Remember what the C# compiler sees is what the preprocessor allows it to. You write code and what the compiler sees may be very very different. a.cs #warning We have a code red class zzz { public static void Main() { } } Compiler Warning a.cs(1,10): warning CS1030: #warning: 'We have a code red' Whenever we want a warning message to be displayed at the time of compiling our code we use #warning. a.cs class zzz { #warning We have a code red public static void Main() { } } Compiler Warning a.cs(3,10): warning CS1030: #warning: 'We have a code red' Unlike the #define, the #warning can be used anywhere in our program. It enables us to add to the messages of the compiler. Also the line number changes from 1 to 3 telling us where the warning occurred. a.cs class zzz { #error We have a code red public static void Main() { } } Compiler Error a.cs(3,8): error CS1029: #error: 'We have a code red' Wherever we have warnings, errors cannot be far behind. The difference is that an error unlike a warning, stops everything in its tracks and does not let the compiler proceed ahead. No exe file is created. Normally an error or warning would be placed in an if statement as follows. a.cs #define vijay #define mukhi class zzz
- 96. { #if vijay &&mukhi #error We have a code red #endif public static void Main() { } } Compiler Error a.cs(6,8): error CS1029: #error: 'We have a code red' The && means and. The if is true if both sides of the && is true. They are in this case. Remove one of the above #defines and the if will be false. a.cs #line 100 "vijay" class zzz { #warning We have a code red public static void Main() { } } Compiler Warning vijay(102,10): warning CS1030: #warning: 'We have a code red' The line directive does two things. One it changes the line number from 1 which is what is should be at the beginning to 100. Thus the warning appears on line 102 now and not 2. Also the file name changes from a.cs to vijay. You have total control over the line number and file name displayed. a.cs #line 100 "vijay" class zzz { public static void Main() { int int; #line 200 "mukhi" char char; } } Compiler Error vijay(104,5): error CS1041: Identifier expected, 'int' is a keyword vijay(104,8): error CS1001: Identifier expected mukhi(200,6): error CS1041: Identifier expected, 'char' is a keyword mukhi(200,10): error CS1001: Identifier expected Line does not only work with the #error or #warning. It affects also the compiler's error line number and file name. You are allowed to have as many #lines as you prefer. 7
- 97. Function Overloading andInheritance Its not what you get but what you give that makes you a richer person. Unfortunately, this little gem is understood only be a few and giving remains largely a one way street. This chapter explains function overloading, the params parameter and inheritance. We start with function overloading. Function Overloading a.cs class zzz { public static void Main() { yyy a = new yyy(); a.abc(10); a.abc("bye"); a.abc("no",100); } } class yyy { public void abc(int i) { System.Console.WriteLine("abc" + i); } public void abc(string i) { System.Console.WriteLine("abc" + i); } public void abc(string i,int j) { System.Console.WriteLine("abc" + i + j); } } Output abc10 abcbye abcno100 The class yyy has three functions, all of them having the same name abc. The distinction between them is in the data types of the parameters. They are all different. In C# we are allowed to have functions with the same name, but having different data types parameters. The advantage is that we call the function by the same name as by passing different parameters, a different function gets called. This feature is called function overloading. All is fine only if the parameter types to the function are different. We do not have to remember a large number of functions by name. The only reason why function overloading works is that C# does not know a function by name, but by its signature. A signature denotes the full name of the function. Thus the name of a function or its signature is the original function name plus the number and data types of its individual parameters. a.cs class zzz
- 98. { public void abc() { } publicint abc() { } public static void Main() { } } Compiler Error a.cs(6,12): error CS0111: Class 'zzz' already defines a member called 'abc' with the same parameter types Here we have two functions abc which differ only in the values they return. As return values do not count in the function signature and the function names are similar, hence the error. a.cs class zzz { static void abc(int i) { } public void abc(int i) { } public void abc(string p) { } public static void Main() { } } Compiler error a.cs(6,14): error CS0111: Class 'zzz' already defines a member called 'abc' with the same parameter types We have 2 abc's, that accept an int and differ only in the addition of a modifier static. They have the same signature as modifiers like static are not considered as part of the function signature. Also, in the next program, we have two abc's with different access modifiers which differ in the parameters, hence signature/name changes causing an error. a.cs class zzz { void abc(int i) { } void abc( out int i) { i = 10; } void abc( ref int i) { } public static void Main() { } }
- 99. Compiler Error Microsoft (R)Visual C# Compiler Version 7.00.9254 [CLR version v1.0.2914] Copyright (C) Microsoft Corp 2000-2001. All rights reserved. a.cs(10,6): error CS0663: 'abc' cannot define overloaded methods which differ only on ref and out a.cs(6,6): (Location of symbol related to previous error) The signature consists of not only the parameter data types, but also the kind of parameters i.e. out ref etc. As function abc takes an int with different modifiers i.e. out etc, the signature on each is different. The signature of a method consists of its name and number and types of its formal parameters. The return type of a function is not part of the signature. No two functions can have the same signature and also non-members cannot have the same name as members. A function/method can be called by four different types of parameters. These are pass by value, reference, output and finally parameter arrays. The parameter modifier is not part of the function signature. Lets now understand what parameter arrays are all about. Params Parameter A method declaration creates a separate declaration space. This means that anything created in a method is lost at the end of the method. a.cs public class zzz { public void abc(int i,string i) {} public void pqr(int i) { string i; } public static void Main() { } } Compiler Error a.cs(3,23): error CS0100: The parameter name 'i' is a duplicate a.cs(6,8): error CS0136: A local variable named 'i' cannot be declared in this scope because it would give a different meaning to 'i', which is already used in a 'parent or current' scope to denote something else Parameter names have to be unique. Also, we cannot have a parameter and a variable created in a function block with the same name. In pass by value, the value of the variable is passed. In the case of ref and out, the address of the reference is passed. a.cs public class zzz { string s = "hi"; public static void Main() { zzz z = new zzz(); z.pqr(); } void pqr() { abc(ref s,ref s); System.Console.WriteLine(s);
- 100. } void abc(ref stringa, ref string b) { System.Console.WriteLine(s); a="no"; System.Console.WriteLine(s); b = "yes"; System.Console.WriteLine(s); s = "maybe"; } } Output hi no yes maybe You are allowed to pass the same ref parameter as many times as you desire. In the function abc the string s has a value of hi. Then by changing the string b to no, we are actually changing the string s to no as s is passed by reference. Variables a and s refer to the same string in memory. Changing one changes the other. Again changing b also changes s as they refer to the same string. Thus variables a, b and s refer to the same string in memory. a.cs public class zzz { public static void Main() { zzz z = new zzz(); z.pqr(); } void pqr() { abc(2,"hi","bye","no"); abc(20,"hi"); abc(2); } void abc(int i , params string [] b) { foreach ( string s in b) System.Console.WriteLine(s + " " + i); } } Output hi 2 bye 2 no 2 hi 20 We will encounter a situation where we would like to pass a variable number of arguments to a function. This is not possible as of now as C# is extremely finicky about the number and type of data we pass to a function. If we pass a string where an int is expected, C# starts screaming like a baby. If we want to pass a variable number of arguments to a function, we have to use a keyword params. This keyword can only be applied to the last parameter. Therefore the variable number of arguments can only come at the end. In the case of function abc, the first parameter has to be an int, the rest of them can be from zero to an infinite number of strings. a.cs public class zzz { public static void Main() {
- 101. } void abc(int i, params string [] b , int j) { } } Compiler Error a.cs(6,18): error CS0231: A params or __arglist parameter must be the last parameter in a formal parameter list The params keyword in this version has to be at the end only as stated above. a.cs public class zzz { public static void Main() { zzz z = new zzz(); z.pqr(); } void pqr() { abc(2,3,4); abc(20,1); abc(2); } void abc(int i , params int [] b) { foreach ( int s in b) System.Console.WriteLine(s + " " + i); } } Output 3 2 4 2 1 20 C# is smart enough if the penultimate parameter and the params have the same data type. The first int is stored in the variable i, the rest are made part of the array b. a.cs public class zzz { void abc(int i , params string [][] b) { } void abc(int i , params string [,] b) { } } Compiler Error a.cs(6,6): error CS0225: The params parameter must be a single dimensional array The data type of the params parameter must be, as the error message states, a single dimensional array. Thus [][] is allowed but not [,]. You are also not allowed to combine the params keyword with ref or out. a.cs public class zzz { public static void Main() { zzz z = new zzz(); z.pqr();
- 102. } void pqr() { string []s = {"hi","bye","no"}; abc(2,s); } void abc(int i , params string [] b) { foreach ( string s in b) System.Console.WriteLine(s + " " + i); } } Output hi 2 bye 2 no 2 You are allowed to pass an array of strings instead of individual strings as parameters. Here s is an array of strings which has been initialized using the short form. Internally when we call the function abc, C# converts the array of strings into individual strings. a.cs public class zzz { public static void Main() { zzz z = new zzz(); z.pqr(); } void pqr() { string [] s1 = {"hi","bye"}; abc(2,s1,"hell"); } void abc(int i , params string [] b) { } } Compiler Error a.cs(11,1): error CS1502: The best overloaded method match for 'zzz.abc(int, params string[])' has some invalid arguments a.cs(11,7): error CS1503: Argument '2': cannot convert from 'string[]' to 'string' Mixing and matching is not allowed in C#. What we assumed C# would do is to add the last string hell to the array of strings s1 or convert s1 to individual strings and then add the string hell to it. Perfectly logical we thought. Only if wishes were horses… Internally before calling the function abc, it collects all the individual parameters and converts it into one big array for the params statement. a.cs public class zzz { public static void Main() { zzz z = new zzz(); z.pqr(); } void pqr() {
- 103. int [] a= {1,2,3}; abc(2,a); System.Console.WriteLine(a[1]); } void abc(int i , params int [] b) { b[1] = 100; } } Output 100 The output produced is proof of concept. The array member a[1] has an initial value of 2 and in the function abc we change it to 100. The original changes, this means that the array is given to the function abc. a.cs public class zzz { public static void Main() { zzz z = new zzz(); z.pqr(); } void pqr() { int a = 10; abc(2,100,a,20); System.Console.WriteLine(a); } void abc(int i , params int [] b) { b[1] = 100; } } Output 10 In this case C# creates an array containing 100 10 and 20. We are changing the second member to 100 which has nothing to do with the variable a. As abc has no knowledge of a, how on earth can abc change the value of the int a? Thus it stays the same. a.cs public class zzz { public static void Main() { zzz z = new zzz(); z.pqr(); } void pqr() { abc(2); abc(2,3); abc(2,3,5,6); } void abc(int i, int j) { System.Console.WriteLine("two ints "+ i + " " + j); } void abc(params int [] a) {
- 104. System.Console.WriteLine("params a"); } } Output params a twoints 2 3 params a Here we are discussing function overloading. C# is extremely bright though partial. It does not like the params statement and treats it like a stepchild. When we call abc with one int, C# can only call the abc that takes a params as a parameter as it matches one int. An array can contain one member. The fun starts with the abc that is being called with two ints. Here we have a dilemma. C# can call the params abc or the abc with two ints. As mentioned earlier, C# treats the params as a second class citizen and therefore chooses the abc with two ints. When there are more than two ints like in the third invocation, C# has no choice but to grudgingly choose the abc with the params. C# chooses the params as a last resort before flagging an error. a.cs using System; class zzz { static void ff(params object[] b) { foreach (object o in b) { Console.Write(o.GetType().FullName + " "); } Console.WriteLine(); } static void Main() { object[] a = {1, "Hello", 123.456}; object o = a; ff(a); ff((object)a); ff(o); ff((object[])o); } } Output System.Int32 System.String System.Double System.Object[] System.Object[] System.Int32 System.String System.Double In the first case we are passing the function ff an array of object that looks like object. We will tell you a little later that all classes are derived from object. The function ff receives an array of objects b. In the foreach we know that the object class has a function GetType that returns an object that looks like Type, which in turn has a function called FullName which returns the name of the type. We see three different types displayed. In the second invocation of ff we are casting a to an object. There is no conversion available from converting an object to an object array i.e. object []. Therefore a one element object [] is created. It's the same case in the third invocation and the last explicitly casts to an object array. a.cs using System; class zzz { static void ff(params object[] b) { Console.WriteLine(b.GetType().FullName); Console.WriteLine(b.Length); Console.WriteLine(b[0]); } static void Main() { object[] a = {1, "Hello", 123.456};
- 105. ff((object)a); } } Output System.Object[] 1 System.Object[] Inheritance a.cs class zzz { public staticvoid Main() { xxx a = new xxx(); a.abc(); } } class yyy { public int i = 10; public void abc() { System.Console.WriteLine("yyy abc"); } public void pqr() { System.Console.WriteLine("yyy pqr"); } } class xxx { } Compiler Error a.cs(6,1): error CS0117: 'xxx' does not contain a definition for 'abc' The class yyy contains 2 functions and one instance variable. The class xxx contains no code and no variables at all. An empty class does not denote any error as we are able to instantiate an object that looks like xxx. The error comes about because the class xxx has no function called abc. However the class yyy has a function abc. Would it not be great if we were allowed to use all the code in the class yyy from xxx. Easier said than done, we guess! a.cs class zzz { public static void Main() { xxx a = new xxx(); a.abc(); } } class yyy { public int i = 10; public void abc() { System.Console.WriteLine("yyy abc"); } public void pqr() {
- 106. System.Console.WriteLine("yyy pqr"); } } class xxx: yyy { } Output yyy abc The error disappears and the abc in yyy gets executed. If after the name of the class you write : yyy i.e. the name of another class, a lot happens at once. xxx is now said to have been derived from yyy. What that means is all the code we wrote in yyy can now be used in xxx. It is if we actually wrote all the code that is contained in yyy in xxx. If we had created an object that looks like yyy, everything that the object could do, now an object that looks like xxx can also do. But we have not written a line of code in xxx. We are made to believe that xxx has one variable i and two functions abc and pqr as yyy contains these two functions. Here we are teaching you the concepts of inheritance where yyy will now be called the base class, xxx the derived class. a.cs class zzz { public static void Main() { xxx a = new xxx(); a.abc(); } } class yyy { public int i = 10; public void abc() { System.Console.WriteLine("yyy abc"); } public void pqr() { System.Console.WriteLine("yyy pqr"); } } class xxx : yyy { public void abc() { System.Console.WriteLine("xxx abc"); } } Compiler Warning a.cs(23,13): warning CS0108: The keyword new is required on 'xxx.abc()' because it hides inherited member 'yyy.abc()' Output xxx abc Nothing in the world stops class xxx from creating a function abc i.e. one with the same name as in the base class . C# simply gives us a warning. When we run a.abc(), C# first checks whether the class xxx (as a looks like xxx) has a function called abc. If it does not, then it will check in the base class. Earlier abc was only available in the base class and hence got executed. Here as it is already there in xxx, it gets called from xxx and not yyy. Remember the derived classes get a first shot at execution, then the base class. The reason being, the base class may have a number of functions and for various reasons you may not be satisfied with what they do. You should have the right to have your copy of the function to be called. In other words the derived classes functions override the ones in the base class.
- 107. a.cs class zzz { public staticvoid Main() { xxx a = new xxx(); a.abc(); } } class yyy { public int i = 10; public void abc() { System.Console.WriteLine("yyy abc"); } public void pqr() { System.Console.WriteLine("yyy pqr"); } } class xxx : yyy { public void abc() { System.Console.WriteLine("xxx abc"); base.abc(); } } Output xxx abc yyy abc What if you want the best of both worlds? You may want to call the base classes abc first and then yours or vice versa. To accomplish this, C# gives you a reserved word, something free called base. The word base can be used in any derived class. It means call the function off the base class. Simple. Thus base.abc will call the function abc from yyy the base class of xxx. a.cs class zzz { public static void Main() { xxx a = new xxx(); a.abc(); } } class yyy { public int i = 10; public void abc() { System.Console.WriteLine("yyy abc"); } public void pqr() { System.Console.WriteLine("yyy pqr"); } } class xxx : yyy
- 108. { public void abc() { System.Console.WriteLine("xxxabc"); base.pqr(); } } Output xxx abc yyy pqr There is only one small change made to the program and that is base.abc is replaced by base.pqr. In this case the function pqr from the class yyy gets called. Base is very general purpose. It lets you access members of the base class from the derived class. You cannot use base in yyy as yyy is not derived from any class. Thus base can only be used in derived classes. a.cs class zzz { public static void Main() { yyy a = new yyy(); a.xyz(); } } class yyy { public void abc() { System.Console.WriteLine("yyy abc"); } } class xxx : yyy { public void xyz() { } } Compiler Error a.cs(6,1): error CS0117: 'yyy' does not contain a definition for 'xyz' In this case, xxx is derived from yyy and not vice versa. Thus xxx can use all the members of yyy. Inheritance does not work backwards. Whatever members xxx comprises do not permeate upwards to yyy. Class xxx may now have a function xyz but it cannot give it to class yyy and thus an error occurs. A class inherits everything from its base class except the constructors and destructors. If a class c is derived from class b, which in turn has been derived from class a, class c inherits all the members declared in class b and also class a. This concept is called transitive. A derived class can inherit all the members of the base class but cannot subtract or remove members off that base class. A derived class can hide members of the base class by creating functions by the same name. The original member in the base class remains unchanged and unaffected by whatever is happening in the derived class. It remains unchanged in the base class, it is simply not visible in the derived class. A class member can either be a static member belonging to the class or an instance member belonging to the instance i.e. accessible through the object and not the class. The default is non-static. A class is also called a data structure. It consists of data members like constants, fields and events and function members like methods, properties, indexers, operators, constructors, static constructors and destructors. A class within a class is called a nested class. Thus we can place 11 different types of entities in a class. Function
- 109. members are theonly members of a class that contain executable code. A class creation creates a new declaration space. All classes derive from object . Object is the mother of all classes. a.cs public class zzz : object { public static void Main() { } } If you do not derive from any class, then the C# compiler automatically adds :object to your class definition. Object, the only class to have this feature is not derived from any class. It is the ultimate base class of all classes in the C# hierarchy. class aa { } class bb : aa { } Class aa is the base class of bb . The documentation however calls aa the direct base class of bb. Thus the base classes of bb are aa and object. a.cs public class zzz { public static void Main() { } } class aa : System.Delegate { } class bb : System.Enum { } class cc : System.Array { } class dd : System.ValueType { } Compiler Error a.cs(7,7): error CS0644: 'aa' cannot inherit from special class 'System.Delegate' a.cs(10,7): error CS0644: 'bb' cannot inherit from special class 'System.Enum' a.cs(13,7): error CS0644: 'cc' cannot inherit from special class 'System.Array' a.cs(16,7): error CS0644: 'dd' cannot inherit from special class 'System.ValueType' You cannot derive a class from the above 4 classes as they are special. a.cs public class zzz { public static void Main() { }
- 110. } class aa { } class bb { } classcc : aa, bb { } Compiler Error a.cs(9,16): error CS0527: 'bb' : type in interface list is not an interface A class can only be derived from one more class . You are not permitted to derive from two or more classes i.e. multiple inheritance is not supported. Thus every class has one and only one base class. a.cs public class zzz { public static void Main() { } } class aa : bb { } class bb : cc { } class cc : aa { } Compiler Error a.cs(13,7): error CS0146: Circular base class definition between 'cc' and 'aa' class aa is derived from bb. Class bb in turn is derived from cc and cc is derived from aa. This results in a circular definition. class aa is derived from bb and cc, as bb is derived from cc. As cc is also derived from aa class, bb also derives from this class which is aa. Thus aa is derived from aa which is a logical impossibility. Equating Objects a.cs class zzz { public static void Main() { yyy a = new yyy(); xxx b = new xxx(); a = b; b = a; } } class yyy { public int i=10; } class xxx {
- 111. public int j=10; } CompilerError a.cs(7,5): error CS0029: Cannot implicitly convert type 'xxx' to 'yyy' a.cs(8,5): error CS0029: Cannot implicitly convert type 'yyy' to 'xxx' C# has a very simple rule. It does not like you to equate different objects to each other. Thus an object that looks like yyy cannot be equated to one that looks like xxx and vice versa. Thus the error. Another example - you cannot take an int and equate it to a string . C# is extremely strict when it comes to dealing with different data types. a.cs class zzz { public static void Main() { yyy a = new yyy(); xxx b = new xxx(); a = b; b = a; } } class yyy { public int i=10; } class xxx : yyy { public int j=10; } Compiler Error a.cs(8,5): error CS0029: Cannot implicitly convert type 'yyy' to 'xxx' There is however one way out. On account of this way, one of the errors disappeared. The only time we are allowed to equate dissimilar data types is when we derive from them. Lets explain this in detail. When we create an instance of yyy by saying new, we are creating two objects at one go, one that looks like yyy and the other that looks like object. All classes in C# are finally derived from object. As xxx is derived from yyy, when we say new xxx, we are creating 3 objects, one that looks like yyy, one that looks like xxx and finally object. Thus when we write a = b, b looks like xxx, yyy and object and as a looks like yyy, there is a match at yyy. Consequence? No error. Even though a and b have the same values, using a we can only access the members of yyy, even though had we used b we could access xxx also. We have devalued the potency of a . The error arises at b = a, because the class yyy is less/smaller than the class xxx . The class xxx has yyy and more. We cannot have a larger class on the right and a smaller class on the left. a only represents a yyy whereas b expects a xxx which is a xxx and yyy. The basic rule is that we can only equate dissimilar objects if they are derived from each other. You can equate an object of a base class to a derived class but not vice versa. a.cs class zzz { public static void Main() { yyy a = new yyy(); xxx b = new xxx(); a = b; b = (xxx) a; }
- 112. } class yyy { public inti=10; } class xxx : yyy { public int j=10; } Though we broke a C# rule on equating objects, we did not get an error because of the cast . A () is called a cast. Within the brackets we put the name of a class. A cast is the great leveler. When we write b = a, C# expects the right hand side of the equal to to be a b i.e. a xxx . Instead it finds a i.e. a yyy . So by applying a cast, we are for the moment converting the yyy object into an xxx. This strategy satisfies the rules of C# on only equating similar objects. Remember it is only for the duration of the line that a becomes a xxx and not a yyy. a.cs class zzz { public static void Main() { yyy a = new yyy(); xxx b = new xxx(); a = (yyy) b; b = (xxx) a; } } class yyy { public int i=10; } class xxx { public int j=10; } Compiler Error a.cs(7,6): error CS0030: Cannot convert type 'xxx' to 'yyy' a.cs(8,6): error CS0030: Cannot convert type 'yyy' to 'xxx' Unfortunately casting works only if one of the two classes is derived from the other. You cannot cast any two objects to each other. a.cs class zzz { public static void Main() { int i = 10; char j = 'A'; i = j; j = i; } } Compiler Error a.cs(8,5): error CS0029: Cannot implicitly convert type 'int' to 'char' We are allowed to convert a char into a int as i = j but not the other way round as j = i.
- 113. 8 Modifiers Access Modifiers Public, Private,Protected and Internal Whenever a class is created by us we want to have the ability to decide who can access certain members of the class. In other words, we would like to restrict access to the members of the class. The basic rule is that members of a class can freely access each other. There is no way one can prevent a function of a particular class from executing another function in the same class. By default though, the same class is allowed complete access but no one else is granted access to the members of the class. The default access modifier is private. a.cs class zzz { public static void Main() { yyy.pqr(); } } class yyy { static void abc() { System.Console.WriteLine(“yyy abc”); } public static void pqr() { System.Console.WriteLine(“yyy pqr”); abc(); } } Output yyy pqr yyy abc Pqr is public and hence anyone is allowed to execute it. abc has no access modifier which makes it private, which is anyway the default. The private modifier has no effect on members of the same class and hence pqr is allowed to call abc. This concept is called member access. a.cs class zzz { public static void Main() { yyy.abc(); } } class yyy { static void abc()
- 114. { System.Console.WriteLine(“yyy abc”); } public staticvoid pqr() { System.Console.WriteLine(“yyy pqr”); abc(); } } Compiler Error a.cs(5,1): error CS0122: ‘yyy.abc()’ is inaccessible due to its protection level abc is private an no one but members of yyy can access it. a.cs class zzz { public static void Main() { yyy.abc(); } } class yyy { protected static void abc() { System.Console.WriteLine(“yyy abc”); } public static void pqr() { System.Console.WriteLine(“yyy pqr”); abc(); } } Compiler Error a.cs(5,1): error CS0122: ‘yyy.abc()’ is inaccessible due to its protection level We have now introduced one more access modifier, protected, which also does not let you access a class from outside. However pqr is allowed to access abc as access modifiers do not effect the same class as mentioned earlier. a.cs class zzz { public static void Main() { xxx.aaa(); } } class yyy { static void abc() { System.Console.WriteLine(“yyy abc”); } public static void pqr() { System.Console.WriteLine(“yyy pqr”); } protected static void xyz() { System.Console.WriteLine(“yyy xyz”); } }
- 115. class xxx :yyy { public static void aaa() { abc(); pqr(); xyz(); } } Compiler Error a.cs(27,1): error CS0122: ‘yyy.abc()’ is inaccessible due to its protection level We are now dealing with derived classes. When we flag a function with the modifier, protected, we are informing C# that only derived classes can access the function. Nobody else can. Thus in function aaa we can call xyz as it is flagged protected, but it cannot be called from anywhere else including Main. The function abc is private and can be called only from the class yyy. Comment out abc(); in aaa and csc will show you no errors. To sum up, we have learnt three concepts. Private means only the same class has access, public means everyone has access and protected lies in between where only derived classes have access. All functions for example reside in a class. The accessibility of that function is decided by the class in which it resides as well as the modifiers on the function. If we are allowed access to a member, we say that the member is accessible, else inaccessible. b.cs internal class yyy { } csc /t:library b.cs This command will produce a library b.dll with one class yyy. a.cs class zzz { public static void Main() { yyy a; } } >csc a.cs /r:b.dll Compiler Error a.cs(5,1): error CS0122: ‘yyy’ is inaccessible due to its protection level We get the above error as the modifier internal means that we can only access yyy from b.dll and not from any other file or program. Never create a component and flag the class internal as no one would be able to use it. Internal means access limited to this program only. Also writing csc a.cs b.cs would not give us any error. a.cs public namespace vijay { class zzz { public static void Main() { } }
- 116. } Compiler Error a.cs(1,8): errorCS1518: Expected class, delegate, enum, interface, or struct Namespaces by default can have no accessibility modifiers at all. They are public by default and we cannot add any other access modifier including public again. a.cs private class zzz { } Compiler Error a.cs(1,1): error CS1527: Namespace elements cannot be explicitly declared as private or protected A class can only be public or internal. It cannot be marked as protected or private. The default is internal. b.cs class yyy { } csc b.cs /t:library Compiler Error fatal error CS2022: Options ‘/out’ and ‘/target’ must appear before source file names Mistake, done on purpose. At times we will forget to tell you that some compiler options like /t and /out must appear before the names of the source files. >csc /t:library b.cs a.cs class zzz { public static void Main() { yyy a; } } >csc a.cs /r:b.dll Compiler Error a.cs(5,1): error CS0122: ‘yyy’ is inaccessible due to its protection level Thus if we want other programs/files to access classes created by us, we must not forget that by default they are marked as internal as explained earlier. Members of a class can have all the modifiers described above and default to private. a.cs class zzz { public static void Main() { } public private void abc() { } }
- 117. Compiler Error a.cs(6,8): errorCS0107: More than one protection modifier You are not allowed more than one access modifier most of the time. The exceptions, we will soon take care off. Predefined types like int, object have no accessibility restrictions. They can be used anywhere and everywhere. b.cs class yyy { public void abc() { } } csc /t:library b.cs a.cs class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } Compiler Error a.cs(5,1): error CS0122: 'yyy' is inaccessible due to its protection level a.cs(6,1): error CS0246: The type or namespace name 'a' could not be found (are you missing a using directive or an assembly reference?) As the class yyy has not been specified by an access modifier, it is by default internal. Even though abc is public, the type enclosing it i.e. yyy is internal and hence no member of yyy can be accessed from outside b.cs. Thus the access modifiers of the class and the members is important. b.cs public class yyy { void abc() { } } >csc /t:library b.cs a.cs class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } Compiler Error a.cs(6,1): error CS0122: ‘yyy.abc()’ is inaccessible due to its protection level Here yyy is accessible as the modifier is public but the function abc is private and hence cannot be accessed by anyone but the class.
- 118. From now onwe will only display a.cs and b.cs as the command line executions of the compiler will remain the same. a.cs remains the same for this program. b.cs public class yyy { internal void abc() { } } Compiler Error a.cs(6,1): error CS0122: ‘yyy.abc()’ is inaccessible due to its protection level Internal means no one from outside the dll can access the function. a.cs class zzz { public static void Main() { ppp a = new ppp(); a.aaa(); } } b.cs public class yyy { protected internal void abc() { } } public class xxx : yyy { void pqr() { abc(); } } public class ppp { public void aaa() { yyy a = new yyy(); a.abc(); } } No error occurs as protected internal means two things. It is either derived classes or classes in the same file that can access abc. Therefore derived class xxx can use it as well as class ppp. What we are trying to get at here is that the containing type decides first the accessibility and then the member modifiers also comes in. Making the class internal and then the members public will in no way allow classes in others files access it. a.cs class zzz { public static void Main() { } }
- 119. class yyy { protected intx; void abc( yyy a , xxx b) { a.x = 1; b.x = 2; } } class xxx : yyy { void pqr( yyy a , xxx b) { a.x = 1; b.x = 2; } } Compiler Error a.cs(20,1): error CS1540: Cannot access protected member ‘yyy.x’ via a qualifier of type ‘yyy’; the qualifier must be of type ‘xxx’ (or derived from it) Class yyy contains a protected member x. To the same class no modifiers make sense. However as x is protected, in the derived class function pqr, we cannot access it through yyy as a.x gives us an error. However b which looks like xxx does not give an error. To check this out, comment out the line a.x=1 in pqr(). This means that we can access the protected members not from an object of the base class, but from the derived class objects only. This is in spite of the fact that x is a member of yyy, the base class. Even so, we still cannot access it. Also we cannot access x from the function Main. a.cs class zzz { public static void Main() { } } class yyy { } public class xxx : yyy { } Compiler Error a.cs(10,14): error CS0060: Inconsistent accessibility: base class ‘yyy’ is less accessible than class ‘xxx’ Between internal and public, public allows greater access to its members. The class yyy is by default internal and xxx which derives from yyy is explicitly made public. We get an error as the derived class yyy has to have an access modifier which allows greater access than the base class access modifier. Here internal is more restrictive than public. a.cs class zzz { public static void Main() { } } public class yyy { } class xxx : yyy { }
- 120. If we reversethe modifiers, i.e. we make yyy public and xxx the derived class internal we get no error. The base class allows more accessibility than the derived class. a.cs class zzz { public static void Main() { } } class yyy { } public class xxx { public yyy f() { return new yyy(); } } Compiler Error a.cs(12,12): error CS0050: Inconsistent accessibility: return type ‘yyy’ is less accessible than method ‘xxx.f()’ The accessibility of yyy is internal which is more restrictive than public. The accessibility of function f is public which is more than that of the type yyy. The error occurred as return values must have greater accessibility than that of the method, which is not true in this case. a.cs class zzz { public static void Main() { } } class yyy { } public class xxx { public yyy a; } Compiler Error a.cs(12,12): error CS0052: Inconsistent accessibility: field type ‘yyy’ is less accessible than field ‘xxx.a’ Rules are rules – they remain the same everywhere. The class yyy or data type yyy is internal. a, an object/field is public which makes it more accessible than yyy which is internal. Hence the error. a.cs class zzz { public static void Main() { } } class yyy { } public class xxx {
- 121. yyy a; } Now weget no error as a has been made private which gives it a lower accessibility than yyy which is internal. Logic is that whatever you create must be more accessible than what you create from. Sealed Classes Sealed is another modifier that applies to classes. aaa is a sealed class. No class can derive from aaa. In another words aaa cannot act as a base class for any class. a.cs public class zzz { public static void Main() { } } sealed class aaa { } class bbb : aaa { } Compiler Error a.cs(10,7): error CS0509: ‘bbb’ : cannot inherit from sealed class ‘aaa’ a.cs public class zzz { public static void Main() { aaa a = new aaa(); System.Console.WriteLine(a.i); a.abc(); } } sealed class aaa { public int i = 9; public void abc() { System.Console.WriteLine(“hi”); } } Output 9 hi The only difference between a sealed class and a non-sealed class is that a sealed class cannot be derived from. Otherwise there is no difference at all. It can contain the same variables, functions etc as a normal class does . A sealed class lets us create classes which no one can derive from. Thus the code in such classes cannot be overridden. Also as the compiler knows this, certain run time optimizations can be performed on a sealed class Constants a.cs public class zzz {
- 122. const int i= 10; public static void Main() { System.Console.WriteLine(i); } } Output 10 A constant or const variable behaves as a variable. We give it an initial value and can use it wherever we can use a variable. a.cs public class zzz { const int i = 10; public static void Main() { i++; System.Console.WriteLine(i); i = 30; } } Compiler Error a.cs(6,1): error CS0131: The left-hand side of an assignment must be a variable, property or indexer a.cs(8,1): error CS0131: The left-hand side of an assignment must be a variable, property or indexer Unlike a variable, we are not allowed to change the value of a const. The change is an assignment statement. a.cs public class zzz { const int i ; public static void Main() { i = 30; System.Console.WriteLine(i); } } Compiler Error a.cs(3,13): error CS0145: A const field requires a value to be provided We have to initialize the const variable at the time of creation. We are not allowed to initialize it later in our program. a.cs public class zzz { const int i = j + 4; const int j = k - 1; const int k = 3; public static void Main() { System.Console.WriteLine(“{0} {1} {2}”,i,j,k); } } Output 6 2 3
- 123. A constant candepend upon another constant. C# is smart enough to realize that to calculate the value of const i, it first needs to know the value of j. j’s value depends upon another const k, whose value is 3. Thus C# first evaluates k to 3 then j becomes 2 i.e. k -1 and finally i takes on the value of j i.e. 2 + 4 resulting in 6. Like classes const’s cannot be circular i.e., they cannot depend upon each other. a.cs public class zzz { const int i = j + 4; const int j = k - 1; const int k = i; public static void Main() { System.Console.WriteLine(“{0} {1} {2}”,i,j,k); } } Compiler Error a.cs(3,11): error CS0110: The evaluation of the constant value for ‘zzz.i’ involves a circular definition The value of the const i depends upon j which in turn depends upon k, which is equal to i. This becomes a circular definition. A const is a variable whose value cannot be changed but whose initial value is compile time determined. a.cs public class zzz { public const aa a = new aa(); public static void Main() { } } public class aa { } Compiler Error a.cs(3,17): error CS0133: The expression being assigned to ‘zzz.a’ must be constant a.cs public class zzz { public const aa a = null; public static void Main() { } } public class aa { } The error vanishes as we are now initializing a to an object which has a value that can be determined at compile time. We cannot ever change the value of a, so it will always be null. Normally we do not have consts as a reference type as they have value only at runtime. As mentioned earlier we can only initialize a const to a compile time value i.e. a value available to the compiler while it is executing. new unfortunately gets executed at runtime and therefore has no value at compile time. This gives us an error. a.cs class zzz
- 124. { public static voidMain() { yyy y = new yyy(); System.Console.WriteLine(y.i); } } class yyy { public const int i = 3; } Compiler Error a.cs(6,26): error CS0176: Static member 'yyy.i' cannot be accessed with an instance reference; qualify it with a type name instead A constant is static by default and we cannot use the instance reference i.e. a name to reference a const. A const has to be static as no one is allowed to make any changes to a const. a.cs class zzz { public static void Main() { } } class yyy { public static const int i = 3; } Compiler Error a.cs(9,25): error CS0504: The constant ‘yyy.i’ cannot be marked static C# does not want us to repeat the obvious over and over again. Just like humans, programming language too have their own quirks. Some other time, perhaps, C# may permit us to write a static before an entity that is already static by default. a.cs class zzz { public static void Main() { System.Console.WriteLine(yyy.i + “ “ + xxx.i); } } class yyy { public const int i = 3; } class xxx : yyy { public const int i = 30; } Compiler Warning a.cs(14,18): warning CS0108: The keyword new is required on ‘xxx.i’ because it hides inherited member ‘yyy.i’ Output 3 30 We can create a const with the same name as another const in the base class. The const of the class xxx i will hide the const i in class yyy for the class xxx only.
- 125. Fields A field tostart with is another word for a variable in a class. There are a large number of generic rules that apply to all members of a class and we will not tire you by repeating them ad nauseam. A variable can never have an uninitialized value in C#. a.cs public class zzz { static int i; static bool j; public static void Main() { System.Console.WriteLine(zzz.i + “ “ + zzz.j ); } } Output 0 False Static variables are initialized when the class is loaded first. An int is given an initial value of zero and a bool False. a.cs public class zzz { int i; bool j; public static void Main() { zzz a = new zzz(); System.Console.WriteLine(a.i + “ “ + a.j ); } } Output 0 False An instance variable is initialized at the time of creation. The keyword new will create an instance of the zzz. It will allocate memory for each of the non static variables and then initialize each of them to their default values. a.cs public class zzz { static int i = j + 10; static int j = i + 1; public static void Main() { System.Console.WriteLine(zzz.i + “ “ + zzz.j ); } } Output 10 11 Outputs make a lot of sense if you understand them in plain simple English. C# always initializes static fields to their initial value after creating them . Variables i and j are thus given a default of zero. Then C# realizes that these variables need to be assigned some values. It does not read all the lines, only one at a time. It will now read the first line and as the variable j has a value of 0, i will get a value of 10. Then at the next line, j is the value of i plus 1. The variable i has a value of 10 and j now becomes 11. As it does not see both lines at the same time, it does not notice the circularity of the above definition. In short, though the above example works, it is frowned upon by the powers to be at C#.
- 126. a.cs public class zzz { inti = j + 10; int j = i + 1; public static void Main() { } } Compiler Error a.cs(3,9): error CS0236: A field initializer cannot reference the nonstatic field, method, or property ‘zzz.j’ a.cs(4,9): error CS0236: A field initializer cannot reference the nonstatic field, method, or property ‘zzz.i’ It does not work for instance variables as the rules of an instance variable are different than that of static. The field initializer of an instance variable has to be determined at the time of creation of the object. The variable j does not have a value at this point in time. It cannot refer to variables of the same instance at the time of creation. Thus we can refer to no instance members to initialize an instance member. Textual order means first come first served. Readonly Fields Fields can be also tagged with the modifier readonly. a.cs public class zzz { public static readonly int i = 10; public static void Main() { System.Console.WriteLine(i); } } Output 10 No errors at all. However, remember if we use a non static variable in a static function we will get an error. a.cs public class zzz { public static readonly int i = 10; public static void Main() { i = 20; System.Console.WriteLine(i); } } Compiler Error a.cs(6,1): error CS0198: A static readonly field cannot be assigned to (except in a static constructor or a variable initializer) You cannot change the value of a readonly field after its being given an initial value. a.cs public class zzz { public static readonly int i ; public static void Main() { }
- 127. } Unlike a const,a readonly field does not have to be initialized at the time of creation. a.cs public class zzz { public static readonly int i ; static zzz() { i = 20; System.Console.WriteLine(“In Const”); } public static void Main() { System.Console.WriteLine(i); } } Output In Const 20 A static readonly field can be initialized in a static constructor also. This is the major difference between a const and a readonly field. a.cs public class zzz { public readonly aa a = new aa(); public static void Main() { } } public class aa { } The same example which gave an error with const does not give an error with readonly. To sum up a readonly is a more generic const and it makes our programs more readable as we refer to a name and not a number. Is 100 more intuitive or priceofcopper easier to understand? The compiler would for reasons of efficiency convert all const’s and readonly variables to the actual values. a.cs public class zzz { public static void Main() { } } public class aa { public int readonly i = 10; } Compiler Error a.cs(9,12): error CS1585: Member modifier ‘readonly’ must precede the member type and name a.cs(9,23): error CS1519: Invalid token ‘=’ in class, struct, or interface member declaration Wherever you can place multiple modifiers, remind yourself that there are rules that decide the order of modifiers, which comes first. Here the readonly modifier precedes the data type int. Once again, no great cosmic law responsible, just a rule that must be remembered.
- 128. a.cs public class zzz { publicstatic void Main() { } } public class aa { public readonly int i = 10; void abc(ref int z) { } void pqr() { abc(ref i); } } Compiler Error a.cs(13,9): error CS0192: A readonly field cannot be passed ref or out (except in a constructor) A readonly field cannot be changed by anyone except a constructor. The function abc expects a ref parameter which if you have forgotten allows you to change the value of the original. Thus C# does not permit a readonly as a parameter to a function that accepts a ref or a out parameters. 9 Virtual Functions - new and override Every breath you take, And every move you make Every bond you break, every step you take I’ll be watching you -Every Breath You Take,Sting New and Override Methods a.cs class zzz { public static void Main() { yyy a = new yyy(); xxx b = new xxx(); yyy c = new xxx();
- 129. a.abc();a.pqr();a.xyz(); b.abc();b.pqr();b.xyz(); c.abc();c.pqr();c.xyz(); } } class yyy { public voidabc() { System.Console.WriteLine(“yyy abc”); } public void pqr() { System.Console.WriteLine(“yyy pqr”); } public void xyz() { System.Console.WriteLine(“yyy xyz”); } } class xxx : yyy { public void abc() { System.Console.WriteLine(“xxx abc”); } public void pqr() { System.Console.WriteLine(“xxx pqr”); } public void xyz() { System.Console.WriteLine(“xxx xyz”); } } Compiler Warning a.cs(30,13): warning CS0108: The keyword new is required on 'xxx.abc()' because it hides inherited member 'yyy.abc()' a.cs(34,13): warning CS0108: The keyword new is required on 'xxx.pqr()' because it hides inherited member 'yyy.pqr()' a.cs(38,13): warning CS0108: The keyword new is required on 'xxx.xyz()' because it hides inherited member 'yyy.xyz()' Output yyy abc yyy pqr yyy xyz xxx abc xxx pqr xxx xyz yyy abc yyy pqr yyy xyz Class xxx derives from class yyy. That makes xxx the derived class and yyy the base class. The class xxx comprises yyy and more. Thus can we not conclude, albeit in broken English that an object that looks like xxx is bigger than one that looks like yyy. In C#, you can equate a smaller object to a bigger object as stated earlier. Lets take the case of object a. It looks like yyy and initialized by creating an object that also looks like yyy. When we call the functions abc and pqr and xyz through the object a obviously it will call them from yyy.
- 130. Object b lookslike xxx, the derived class. It is initialized to an object that looks like xxx. When we call abc, pqr and xyz through b, it calls abc, pqr and xyz from xxx. Object c again looks like yyy but it is now initialized to an object that looks like xxx which does not give an error as explained earlier. However there is no change at all in the output and the behavior is identical to that of object a. Hence initializing it to an object that looks like yyy or xxx does not seem to matter. a.cs class zzz { public static void Main() { yyy a = new yyy(); xxx b = new xxx(); yyy c = new xxx(); a.abc();a.pqr();a.xyz(); b.abc();b.pqr();b.xyz(); c.abc();c.pqr();c.xyz(); } } class yyy { public void abc() { System.Console.WriteLine(“yyy abc”); } public void pqr() { System.Console.WriteLine(“yyy pqr”); } public void xyz() { System.Console.WriteLine(“yyy xyz”); } } class xxx : yyy { public override void abc() { System.Console.WriteLine(“xxx abc”); } public new void pqr() { System.Console.WriteLine(“xxx pqr”); } public void xyz() { System.Console.WriteLine(“xxx xyz”); } } Compiler Error a.cs(30,22): error CS0506: ‘xxx.abc()’ : cannot override inherited member ‘yyy.abc()’ because it is not marked virtual, abstract, or override We get an error because we have added two new modifiers, new and override to the functions. The error says to add the modifier virtual to the functions in the base class. a.cs class zzz { public static void Main()
- 131. { yyy a =new yyy(); xxx b = new xxx(); yyy c = new xxx(); a.abc();a.pqr();a.xyz(); b.abc();b.pqr();b.xyz(); c.abc();c.pqr();c.xyz(); } } class yyy { public virtual void abc() { System.Console.WriteLine(“yyy abc”); } public virtual void pqr() { System.Console.WriteLine(“yyy pqr”); } public virtual void xyz() { System.Console.WriteLine(“yyy xyz”); } } class xxx : yyy { public override void abc() { System.Console.WriteLine(“xxx abc”); } public new void pqr() { System.Console.WriteLine(“xxx pqr”); } public void xyz() { System.Console.WriteLine(“xxx xyz”); } } Output yyy abc yyy pqr yyy xyz xxx abc xxx pqr xxx xyz xxx abc yyy pqr yyy xyz There is a single subtle change in the workings of the objects c only and not a and b. Adding the virtual modifier has made all the difference. The difference is in the object c. c looks like the base class yyy but is initialized to an object that looks like the derived class xxx. C# remembers this fact. When we execute c.abc(), C# remembers that object c was initialized by a xxx object and hence it first goes to class xxx. Here the function has a modifier override which in English means, forget about the data type of c which is yyy, call abc from xxx as it overrides the abc of the base class. The override modifier is needed as the derived class functions will get first priority and be called. We mean to override the abc of the base class. We are telling C# that this abc is similar to the abc of the base class. New has the exactly the opposite meaning. The function pqr has the new modifier. C.pqr() calls pqr from yyy and not xxx. New means that the function pqr is a new function and it has absolutely nothing to do with the pqr
- 132. in the baseclass. It may have the same name pqr as in the base class, but that is only a coincidence. As c looks like yyy, the pqr of yyy gets called even though there is a pqr in xxx. When we do not write any modifier, then it is assumed that we wrote new. Thus every time we write a function, C# assumes it has nothing to do with the base class. These modifiers can only be used if the function in the base class is a virtual function. To us virtual means that the base class is granting us permission to call the function from the derived class and not the base class. We have however one caveat, we have to add the modifier override if our derived class function has to be called. a.cs class zzz { public static void Main() { yyy a = new xxx(); yyy b = new vvv(); xxx c = new vvv(); a.abc();a.pqr();a.xyz(); b.abc();b.pqr();b.xyz(); c.abc();c.pqr();c.xyz(); } } class yyy { public void abc() { System.Console.WriteLine(“yyy abc”); } public virtual void pqr() { System.Console.WriteLine(“yyy pqr”); } public virtual void xyz() { System.Console.WriteLine(“yyy xyz”); } } class xxx : yyy { public virtual void abc() { System.Console.WriteLine(“xxx abc”); } public new void pqr() { System.Console.WriteLine(“xxx pqr”); } public override void xyz() { System.Console.WriteLine(“xxx xyz”); } } class vvv : xxx { public override void abc() { System.Console.WriteLine(“vvv abc”); } public void xyz() { System.Console.WriteLine(“vvv xyz”); } }
- 133. Output yyy abc yyy pqr xxxxyz yyy abc yyy pqr xxx xyz vvv abc xxx pqr xxx xyz Pretty long example. To reiterate, we are allowed to initialize a base object to a derived object. The other way around will generate an error. This leads to an object of a base class being initialized to an object of the derived class. The question that now springs to mind is, which function will get called. The one from the base class or the derived class. If the base class object declared the function virtual and the derived class used the modifier override, the derived class function will get called. Otherwise the base class function will get executed. Thus for virtual functions, the data type created is decided at run time. All functions not tagged with virtual are non virtual, and the function to be called is decided at compile time, depending upon the static data type of the object. If the object is initialized to the same data type, none of the above apply. Whenever we have a mismatch, we need rules to resolve the mismatch. Thus we can land up with a situation where an object to a base class can call a function in the derived class. The object a looks like yyy but is initialized to the derived class xxx. a.abc(), first looks into the class yyy. Here it checks whether the function abc is virtual. The answer is an emphatic no and hence everything stops and the function abc gets called from class yyy. a.pqr does the same thing, but the function now is virtual in class yyy. Thus C# looks at the class yyy, the one it was initialized to. Here pqr is flagged with the modifier new. This means that pqr is a new function which has nothing to do with the one in the base class. They only accidentally share the same name. Thus as there is no function called pqr (as it is a new pqr) in the derived class, the one from base class gets called. In the case of a.xyz(), the same steps are followed again, but in the class yyy, we meet the modifier override, which overrides the function in the base class. We are telling C# to call this function in class xxx and not the one in the base class yyy. The object b which also looks like class yyy, is now initialized with an object that looks like vvv and not xxx like before. As abc is non virtual it gets called from yyy. In the case of function pqr, C# now looks into class vvv. Here it sees no function pqr and hence now looks into class xxx. Thus the above rules repeat and it gets called from class yyy. In the case of b.xyz, in class vvv, it is marked new by default and hence this function has nothing to do with the one in class yyy. Hence the one from vvv does not get called but the one from class yyy where it specifies override. public override void xyz() { System.Console.WriteLine(“vvv xyz”); } If we change xyz in class vvv to the above, i.e. we change new to override, the xyz of vvv will get called. The last object c looks like xxx but is now initialized to an object that looks like the derived class vvv. c.abc(), first looks into class xxx where it is marked as virtual. Remember abc is non virtual in class yyy but virtual in xxx. From now on, the function abc is virtual in vvv also but not in class yyy. Virtual is like water, it flows downwards not upwards. As abc is virtual, we now look into class vvv. Here it is marked override and hence abc gets called from class vvv. In the case of pqr, pqr is marked virtual in class yyy and new in xxx, but as there is no function pqr in vvv, none of the modifiers matter at all. Thus it gets called from class xxx. Lastly for function xyz, in class vvv it is marked new. Hence it has no connection with the xyz in class xxx and thus function xyz gets called from xxx and not yyy. a.cs class zzz { public static void Main()
- 134. { } } class yyy { public virtualvoid pqr() {} } class xxx : yyy { public new void pqr() {} } class vvv : xxx { public override void pqr() { } } Compiler Error a.cs(17,22): error CS0506: ‘vvv.pqr()’ : cannot override inherited member ‘xxx.pqr()’ because it is not marked virtual, abstract, or override We get an error as the function pqr in class xxx is marked new. This means that it hides the pqr of class yyy. Form the viewpoint of class vvv, xxx does not supply a function called pqr. The pqr in class xxx has nothing to do with the pqr in yyy. This means that the function pqr of xxx does not inherit the virtual modifier from the function pqr of class yyy. This is what the compiler is complaining about. As the function pqr in xxx has no virtual modifier, in vvv we cannot use the modifier override. You can, however, use the modifier new and remove the warning. a.cs class zzz { public static void Main() { yyy a = new vvv(); a.pqr(); xxx b = new vvv(); b.pqr(); } } class yyy { public virtual void pqr() { System.Console.WriteLine(“yyy pqr”); } } class xxx : yyy { public virtual new void pqr() { System.Console.WriteLine(“xxx pqr”); } } class vvv : xxx { public override void pqr() { System.Console.WriteLine(“vvv pqr”); } } Output yyy pqr
- 135. vvv pqr and ifwe remove the override modifier from pqr in vvv we get public void pqr() { System.Console.WriteLine(“vvv pqr”); } Output yyy pqr xxx pqr That’s the problem with virtual functions. We call them the cause of migraine. The answers are always different from what you expect. Object a looks like a yyy but is initialized to the derived class vvv. As pqr is virtual, C# now looks into class vvv. However before looking into the class, it realizes that in class xxx, pqr is new. This cuts of all connection with the pqr in yyy. Thus the word new is preceded with virtual, otherwise the override modifier would give us an error in class vvv. As pqr in class xxx is new function, having nothing to do with the class yyy, class vvv inherits a new which also has nothing to do with the class yyy. The pqr in class vvv is related to the pqr of class xxx and not of class yyy. Thus the pqr of class yyy gets called. In the second case object b looks like class xxx now but is initialized to an object of class vvv. C# first looks at class xxx. Here pqr is new and virtual, which makes it a unique function pqr. Unfortunately, the pqr in vvv has the override modifier which sees to it that the pqr of vvv hides the pqr of xxx. This calls the pqr of vvv instead. If we remove the override modifier from pqr in class vvv, the default is new, that cuts off the umbilical cord from the pqr of xxx. Thus, as it is, a new function, the pqr of xxx gets called. A virtual function cannot be marked by the modifiers static, abstract or override. A non virtual function is said to be invariant. This means that the same function gets called always, irrespective of whether one exists in the base class or derived class. In a virtual function the run-time type of the instance decides on which function to be called and not the compile-time type as is in the case of non virtual functions. For a virtual function there exists a most derived implementation which gets always gets called. a.cs class zzz { public static void Main() { yyy a = new vvv(); xxx b = new vvv(); www c = new vvv(); vvv d = new vvv(); a.pqr();b.pqr();c.pqr();d.pqr(); } } class yyy { public virtual void pqr() { System.Console.WriteLine(“yyy pqr”); } } class xxx : yyy { public override void pqr() { System.Console.WriteLine(“xxx pqr”); } } class www : xxx {
- 136. public virtual newvoid pqr() { System.Console.WriteLine(“www pqr”); } } class vvv : www { public override void pqr() { System.Console.WriteLine(“vvv pqr”); } } Output xxx pqr xxx pqr vvv pqr vvv pqr One last explanation of virtual with a slightly more complex example involving 4 classes. The first line in Output, xxx pqr, is the result of the statement a.pqr();. We have the function pqr as virtual in class yyy. Hence, when using new, we now proceed to class xxx and not vvv as explained earlier. Here, pqr has an override and C# knows that class www inherits this function pqr. In class www, as it is marked as new, C# will now backtrack and not proceed further to class vvv. Hence the function pqr gets called from class xxx as shown in the output. public override void pqr() { System.Console.WriteLine(“www pqr”); } If we change the function pqr in class www to override, then C# will proceed to class vvv and call the pqr of class vvv as it overrides the pqr of www. Remove the override from pqr in class vvv and the function will get called from class www as the default is new. For object b, everything remains the same as object a, because it overrides the pqr of class yyy. Restoring back the defaults, lets look at the third line. Object c looks like www. In class www, pqr is a new and hence it has nothing to do with the earlier pqr functions. In class vvv, we are overriding the pqr of class www and hence the pqr of vvv gets called. Remove the override and then it will get called from class www. The object d thankfully follows none of the above rules as the left and the right of the equal to sign are the same data types. An override method is a method that has the override modifier included on it. This introduces a new implementation of a method. You cannot use the modifiers new, static or virtual along with override. However abstract is permitted. a.cs class zzz { public static void Main() { yyy a = new xxx(); a.pqr(); yyy b = new www(); b.pqr(); } } class yyy { public virtual void pqr()
- 137. { System.Console.WriteLine(“yyy pqr”); } } class xxx: yyy { public override void pqr() { base.pqr(); System.Console.WriteLine(“xxx pqr”); } } class www : xxx { public override void pqr() { base.pqr(); System.Console.WriteLine(“www pqr”); } } Output yyy pqr xxx pqr yyy pqr xxx pqr www pqr Using the keyword base, we can access the base class functions. Here whether pqr is virtual or not, it is treated as non virtual by the keyword base. Thus the base class pqr will always be called. The object a knows that pqr is virtual. When it goes to yyy, it sees base.abc and hence it calls the pqr of yyy. In the second case, it first goes to class vvv, here it calls the base.abc, i.e. the function pqr of class xxx, which in turn calls function pqr in class yyy. a.cs class zzz { public static void Main() { yyy a = new xxx(); a.pqr(); } } class yyy { public virtual void pqr() { System.Console.WriteLine(“yyy pqr”); } } class xxx : yyy { public override void pqr() { ((yyy)this).pqr(); System.Console.WriteLine(“xxx pqr”); } } Output Unhandled Exception: StackOverflowException.
- 138. No amount ofcasting will stop the infinite loop. Thus even though this is being cast to a yyy, it will always call pqr from xxx and not yyy. Hence you see no output a.cs class zzz { public static void Main() { yyy a = new www(); xxx b = new www(); a.pqr();a.xyz(); b.pqr();b.xyz(); } } class yyy { public virtual void pqr() { System.Console.WriteLine(“yyy pqr”); } public virtual void xyz() { System.Console.WriteLine(“yyy xyz”); } } class xxx : yyy { private new void pqr() { System.Console.WriteLine(“xxx pqr”); } public new void xyz() { System.Console.WriteLine(“xxx xyz”); } } class www : xxx { public override void pqr() { System.Console.WriteLine(“www pqr”); } public void xyz() { System.Console.WriteLine(“www xyz”); } } Output www pqr yyy xyz www pqr xxx xyz More virtual functions, more complications in life. Let us embark on a thousand mile journey with a single step. When we have a statement a.pqr, C# starts at the class yyy as usual, sees virtual, then goes to class xxx. Here pqr is private and hence its scope is limited to class xxx. The modifier new is thus ignored. C# now goes to class www where the pqr overrides the pqr of class yyy and thus we see www pqr. If we remove the override modifier from function pqr in class www, as it is a new function, it will now be called from class yyy. a.xyz calls pqr from yyy due to the explanation given many times before i.e. they are all news.
- 139. b.pqr knows thatpqr in class xxx is private. The scope of this pqr does not extend to class www, the derived class. As the pqr in class www overrides the pqr of class xxx, the one from class www gets called as it overrides the one from the base class yyy. If you change the override to new, then the pqr in www is a new one and has nothing to do with the one from yyy. This calls pqr from the base class yyy. b.xyz calls it from class xxx. This has already been explained at least a trillion times earlier. Abstract Classes a.cs public class zzz { public static void Main() { new aaa(); } } abstract class aaa { } Compiler Error a.cs(5,1): error CS0144: Cannot create an instance of the abstract class or interface ‘aaa’ The keyword abstract can be written before a class. It is called a class modifier. An abstract class cannot be instantiated by the keyword new. We cannot create an object that looks like aaa. Thus we cannot use the class and for all practical purposes the class is useless to us. a.cs public class zzz { public static void Main() { new aaa(); } } abstract class aaa { public int i; public void abc() { } } The error remains the same. Since we have used the modifier abstract in front of the class we cannot use new. Had we removed the modifier abstract, all would be fine. This program is to show that an abstract class can contain variables and functions. a.cs public class zzz { public static void Main() { new bbb(); } } abstract class aaa { public int i; public void abc() { } }
- 140. class bbb :aaa { } We can derive a class bbb from an abstractc class aaa. Thus creating an object that looks like bbb does not give us any error. We cannot yet use aaa directly. a.cs public class zzz { public static void Main() { new bbb(); } } abstract class aaa { public void pqr(); public int i; public void abc() { } } class bbb : aaa { } Compiler Error a.cs(10,13): error CS0501: ‘aaa.pqr()’ must declare a body because it is not marked abstract or extern In a abstract class we have added a function prototype. An extern or abstract function implies that the actual definition or code is created somewhere else. A function prototype in a abstract class must also be declared abstract as per the rules of C#. a.cs public class zzz { public static void Main() { new bbb(); } } abstract class aaa { abstract public void pqr(); public int i; public void abc() { } } class bbb : aaa { } Compiler Error a.cs(16,7): error CS0534: ‘bbb’ does not implement inherited abstract member ‘aaa.pqr()’ After declaring pqr abstract in aaa, we get another error. We are not allowed to create objects that look like bbb because it is derived from aaa, an abstract class which has one abstract function. We have to implement this abstract function pqr in bbb . a.cs
- 141. public class zzz { publicstatic void Main() { new bbb(); } } abstract class aaa { abstract public void pqr(); public int i; public void abc() { } } class bbb : aaa { public void pqr() { } } Compiler Warning a.cs(18,13): warning CS0114: ‘bbb.pqr()’ hides inherited member ‘aaa.pqr()’. To make the current method override that implementation, add the override keyword. Otherwise add the new keyword. Compiler Error a.cs(16,7): error CS0534: ‘bbb’ does not implement inherited abstract member ‘aaa.pqr()’ Now we are really lost. The error clearly says that both classes aaa and bbb have a similar function called pqr. Whenever a class (bbb) derives from another class (aaa) and they both have a function with the same name, an error results. The only way out is for the derived class bbb to explicitly add a keyword override to the function definition. This tells the compiler that we know that the base class has a function of the same name and we want the pqr of bbb to be called, not aaa’s . It is more of a caution exercised by the compiler so that you do not inadvertently override functions of the base class. C# has a large number of such cautions that make you think. a.cs public class zzz { public static void Main() { new bbb(); } } abstract class aaa { abstract public void pqr(); public int i; public void abc() { } } class bbb : aaa { public override void pqr() { } } The warning vanishes after using the keyword override. a.cs
- 142. public class zzz { publicstatic void Main() { new bbb(); } } abstract class aaa { abstract public void pqr(); public int i; public void abc() { } } class bbb : aaa { public override int pqr() { } } Compiler Error a.cs(18,21): error CS0508: ‘bbb.pqr()’: cannot change return type when overriding inherited member ‘aaa.pqr()’ When you are overriding an abstract function from a derived class, you cannot change the parameters passed to it or the return type. If the abstract class has 5 functions, the class derived from it should also implement the same 5 functions without changing return type and/or parameters passed to it. a.cs public class zzz { public static void Main() { new bbb(); } } abstract class aaa { abstract public void pqr(); abstract public void pqr1(); abstract public void pqr2(); public int i; public void abc() { } } class bbb : aaa { public override void pqr() { } } Compiler Error a.cs(18,7): error CS0534: ‘bbb’ does not implement inherited abstract member ‘aaa.pqr1()’ a.cs(18,7): error CS0534: ‘bbb’ does not implement inherited abstract member ‘aaa.pqr2()’ The error goes away if we implement functions pqr1 and pqr2 in bbb.
- 143. An abstract classimplies that the class is incomplete and cannot be directly used. It can only be used as a base class for other classes to derive from. Hence we get a error if we use new on an abstract class. If we do not initialize a variable in an abstract class, it will have a value of 0 which is what the compiler kept warning us about. We can initialize i to any value we want. The variables have the same use and meaning in an abstract class like any other class. Whenever a class is incomplete i.e. we do not have the code for certain functions, we make those functions abstract and the class abstract . This enables us to compile the class without errors. Other classes can then derive from our incomplete class but they have to implement the abstract i.e. our incomplete functions. Abstract thus enables us to write code for part of the class and allows the others to complete the rest of the code. a.cs public class zzz { public static void Main() { } } class aaa { abstract public void pqr(); public int i = 20; public void abc() { } } Compiler Error a.cs(9,22): error CS0513: ‘aaa.pqr()’ is abstract but it is contained in nonabstract class ‘aaa’ If a class has even one abstract function, then the class has to be declared abstract. An abstract method cannot also use the modifiers static or virtual. Only in the abstract class can we have one abstract function. Anyone who implements from an abstract class has to write the code for its function. By default the modifier new gets added, which makes it a new/different function. a.cs class zzz { public static void Main() { } } abstract class yyy { public abstract void abc(); } class xxx : yyy { public override void abc() { base.abc(); } } Compiler Error a.cs(15,1): error CS0205: Cannot call an abstract base method: ‘yyy.abc()’ Like the Sting number, C# is always watching every step you take and every move you make. Like a hawk. You cannot call the function abc from the base class as it does not carry any code along with it and has been declared abstract. Common sense prevails and C# does not allow you to call a function that has no code.
- 144. a.cs class zzz { public staticvoid Main() { yyy a = new www(); xxx b = new www(); a.abc();b.abc(); } } class yyy { public virtual void abc() { System.Console.WriteLine(“yyy abc”); } } abstract class xxx : yyy { public abstract new void abc(); } class www : xxx { public override void abc() { System.Console.WriteLine(“www abc”); } } Output yyy abc www abc a.abc() will first peek into the class yyy. Here it finds function abc tagged as virtual. How many times have we repeated the above lines? Countless times. C# will then toddle over to class xxx. Here it finds to its dismay that the function abc is abstract i.e. there is no code for abc and also that it is a new function, thus severing all links with the base class. Activity stops and all hell breaks loose and the function abc from yyy gets executed. In the case of b.abc(), as the function is new, the links to the base class are broken, we have no choice but to call the function from www as it says override. We cannot replace the modifier new with the keyword override for function abc in abstract class xxx . Compiler Error a.cs(21,7): error CS0534: ‘www’ does not implement inherited abstract member ‘xxx.abc()’ If we replace the override keyword with new in class www, we will get an error as there is no code for the function abc. Remember the abc of yyy has nothing to do at all with that of xxx and www. Virtual functions run slower than non virtual functions and it is obvious that an abstract class cannot be sealed. a.cs public class zzz { public static void Main() { } } sealed abstract class aaa { public abstract void abc(); } Compiler Error
- 145. a.cs(7,23): error CS0502:'aaa' cannot be both abstract and sealed 10 Properties and Indexers Properties Properties are a natural extension to fields. Very few programming languages support the notion of a property. Unlike a variable, a property is not stored in a memory location. It is made up of functions. Thus even though a property and a field share the same syntax a property has the advantage that code gets called. When we initialize a variable, no code in our class gets called. We are not able to execute any code for a variable access or initialization at all. In the case of a property, we can execute tons of code. This is one singular reason for the popularity of a product like Visual Basic - the use of properties. One simple example is setting the value of a variable. If it is through a variable, we have no control over the value used. If the same access is through a property, the programmer has no inkling of whether it is a property or a variable, we can build range checks to make sure that the variable does not cross certain bounds. Lets start by creating a simple property. A property is a member of a class. It behaves like a variable for the user. a.cs public class zzz { public static void Main() { } } public class aa { public int ff { } } Compiler Error a.cs(9,12): error CS0548: ‘aa.ff’ : property or indexer must have at least one accessor We have tried to create a property called ff which is of type int. We get an error because a property is used either on the left or the right of an equal to sign. If we had created a variable ff, we would like to write a statement as gg = ff + 9. Here ff should return some value which is of the data type int. a.cs public class zzz { public static void Main() { aa a = new aa(); int gg = a.ff + 9; System.Console.WriteLine(gg); } }
- 146. public class aa { publicint ff { get { System.Console.WriteLine(“in get”); return 12; } } } Output in get 21 A property should have at least one accessor, in our case, a get as we want to read the value of the property. Thus a.ff calls the get accessor which returns an int, in this case 12. If we did not have access to the code of the class aa, we would have assumed ff to have been a variable. a.cs public class zzz { public static void Main() { aa a = new aa(); a.ff = 19; System.Console.WriteLine(a.ff); } } public class aa { public int ff { get { System.Console.WriteLine(“in get”); return 12; } set { System.Console.WriteLine(value); } } } Output 19 in get 12 A variable can also be used on the left-hand side of the equalto sign. In this case we are writing or changing the value of the variable. We are passing it some value. If it is a property, ff in our case, a.ff = 19 will call the accessor set. The set accessor has a free variable available in it called value. It gets created automatically, we do not create this variable. In our case, this has the value 19, which we are displaying in WriteLine. Then to display the value of the property ff, the get needs to be called again. The get always returns the same answer as the set does not store the value of the variable anywhere. To resolve this issue, we do the following. a.cs public class zzz { public static void Main() {
- 147. aa a =new aa(); a.ff = 19; System.Console.WriteLine(a.ff); } } public class aa { int f1; public int ff { get { System.Console.WriteLine(“in get”); return f1; } set { System.Console.WriteLine(“in set “ + value); f1 = value; } } } Output in set 19 in get 19 To implement a property in real life, we create a public variable which will hold the value of the property. This variable f1 will have the same data type as the property i.e. an int in our case. In the get, we return f1 and in the set we initialize f1 to value. This is the simplest case possible. The reason we use a property and not a variable is because if we change the value of a variable/field, then code in our class is not aware of the change. Also we have no control over what values the variable will contain. The user can change them to whatever he/she likes and we cannot implement range checks on the variable. Also the user may want to associate some action with the changes in the value of the variable. Using a property, reading or writing to the variable also can be monitored. a.cs public class zzz { public static void Main() { aa a = new aa(); a.ff = 19; System.Console.WriteLine(a.ff); } } public class aa { int f1; public int ff { get { System.Console.WriteLine(“in get”); return f1; } } } Compiler Error a.cs(6,1): error CS0200: Property or indexer ‘aa.ff’ cannot be assigned to — it is read only
- 148. You are allowedto declare a property readonly by omitting the set accessor. No one is now allowed to change the value of the property. It now behaves as a const or readonly field. a.cs public class zzz { public static void Main() { aa a = new aa(); a.ff = 19; } } public class aa { int f1; public int ff { set { System.Console.WriteLine(“in set “ + value); f1 = value; } } } Output in set 19 Theoretically, you can have a property which is write only i.e. only with a set accessor. With set, you can change the value of ff but it is of limited use because you can never access the value of ff. A property differs from a field by ending with {}. a.cs public class zzz { public static void Main() { } } public class aa { public int ff { set { } } public int ff { get { } } } Compiler Error a.cs(12,12): error CS0102: The class ‘aa’ already contains a definition for ‘ff’ You cannot create a property in 2 separate bits and pieces. It has to be in one whole. This is part of the syntax. The above creates two properties, both called ff, the first one being write only, the second, read only. The compiler tells you that you cannot create two properties by the same name. a.cs public class zzz {
- 149. public static voidMain() { } } public class aa { private int ff; public int ff { get { } set { } } } Compiler Error a.cs(10,12): error CS0102: The class ‘aa’ already contains a definition for ‘ff’ You obviously cannot have a property and variable with the same name. The compiler would not know whether to invoke the property or the field. They both are stored in the same namespace. a.cs class zzz { public static void Main() { yyy.i = 20; System.Console.WriteLine(yyy.i); } } class yyy { public static int i { get { System.Console.WriteLine(“get”); return 10; } set { System.Console.WriteLine(“set “ + value); } } } Output set 20 get 10 The rules of static apply to properties also. Like variable we access them using the class and not the instance. Everything that we have learned about static in the past applies to properties also. a.cs class zzz { public static void Main() { yyy a = new yyy(); a.i = 100; System.Console.WriteLine(a.i); } }
- 150. abstract class xxx { publicabstract int i { get ; set ; } } class yyy : xxx { public override int i { get { System.Console.WriteLine(“get”); return 10; } set { System.Console.WriteLine(“set “ + value); } } } Output set 100 get 10 The abstract property i in class xxx carries no code at all. The get and set accessors are simply represented by a semicolon. In the derived class, we must implement both the get and the set accessors. If we do not use the override keyword, it is new. We hope you have finally understood new and override. a.cs class zzz { public static void Main() { } } abstract class xxx { public abstract int i { get ; } } class yyy : xxx { public override int i { get { } set { } } } Compiler Error a.cs(20,1): error CS0546: ‘yyy.i.set’: cannot override because ‘xxx.i’ does not have an overridable set accessor
- 151. In class xxx,the abstract property has only a get accessor. In the derived class we are implementing both the get and the set. The original never ever had a set. This is unacceptable to the compiler. Thus we have no choice but to implement only the accessors that are present in the original. A get accessor can be viewed as a method which returns a value but accepts no parameters. a.cs class yyy { public void i { } } Compiler Error a.cs(3,13): error CS0547: ‘i’ : property or indexer cannot have void type It makes no sense for an accessor to have a void type as a variable cannot be of type void. Void literally means ‘I do not know the type’ or no type at all. a.cs class yyy { public int i { set { return 10; } } } Compiler Error a.cs(6,2): error CS0127: Since ‘yyy.i.set’ returns void, a return keyword must not be followed by an object expression A set accessor can be viewed as function which returns void but accepts one parameter which stands for the value of the property. Thus a set cannot return a value. If we remove the 10, we will not get an error. a.cs class zzz { public static void Main() { } public int i { set { value = 20; } } } The reserved variable value in the set can be changed at will. Though, understanding why anyone would want to do such dumb stuff is beyond us. a.cs class zzz { public static void Main() { } public int i { set
- 152. { int value; } } } Compiler Error a.cs(9,6):error CS0136: A local variable named ‘value’ cannot be declared in this scope because it would give a different meaning to ‘value’, which is already used in a ‘parent or current’ scope to denote something else We cannot however create a variable value as it will clash with the variable value which is already present by default in the set. a.cs class zzz { public static void Main() { yyy a = new yyy(); a.i = 10; xxx b = new xxx(); ((yyy)b).i = 20; b.i = 10; } } class yyy { public int i { set { } } } class xxx : yyy { public int i { get { return 10; } } } Compiler Error a.cs(9,1): error CS0200: Property or indexer ‘xxx.i’ cannot be assigned to — it is read only In the class yyy, the property i has only the set accessor. In the class xxx which derives from yyy, we have implemented only the get accessor. The property i in class xxx hides the i of yyy. They do not add up. What we are trying to say is that both these properties are independent of each other. What we had thought C# would have done is, taken the set from one class and added it to the second. However, that does not make sense. It treats them independently. If we want to use the property of the class yyy, then we need to explicitly cast it as we have done for b. Thus the property i of class yyy gets hidden but can be accessed. A property is not necessarily slower than a variable. A variable access normally initializes some memory, whereas a property executes a method. This is not necessarily slower as at times, C# will rewrite your property methods to memory accesses. This is called inlining of code. Except for minor differences, all that we mentioned about virtual, abstract and new apply also to a property. The difference is, if the original property has a get and a set, the derived class will only implement a set or a get. Indexers An indexer lets us access members of a class as if it were an array. a.cs
- 153. public class zzz { publicstatic void Main() { yyy a = new yyy(); a[1] = 10; } } public class yyy { } Compiler Error a.cs(6,1): error CS0021: Cannot apply indexing with [] to an expression of type ‘yyy’ We have created an object a that looks like yyy. The object a, in no sense of the word is an array. We are assuming that a is an array and we’ve used the array syntax a[], hence it gives us an error. a.cs public class zzz { public static void Main() { yyy a = new yyy(); a[1] = 10; } } public class yyy { public int this[int i] { set { System.Console.WriteLine(“in get “ + value + “ “ + i); } } } Output in get 10 1 We’ve added a few lines to have the array notation work with an object that looks like yyy. To implement indexers, we need to create a special property called this. This is a reserved word. As of now, we have a parameter i (an int) in the square brackets. When we did properties earlier,we learnt that a set gets called whenever we want to initialize or set a variable. Within the set accessor we have a special variable called value which stores the value passed to the set, in this case 10. The variable i will hold the value 1 as the array parameter is 1. This is how we implement arrays when there are none. a.cs public class zzz { public static void Main() { yyy a = new yyy(); System.Console.WriteLine(a[1]); } } public class yyy { public int this[int i] { set { System.Console.WriteLine(“in get “ + value + “ “ + i);
- 154. } get { System.Console.WriteLine(“in set “+ i); return 23; } } } Output in set 1 23 The rules binding properties are applicabe to indexers too. When you want to read the value of a[1], the get gets called. The major difference between properties and indexers is that when you implement the code for indexers you have to understand that the get and set get called with a variable which is the array parameter value. The code will have to understand array simulation. a.cs public class zzz { public static void Main() { yyy a = new yyy(); a[“hi”] = 30; System.Console.WriteLine(a[“hi”]); } } public class yyy { public int z; public int this[string i] { set { System.Console.WriteLine(“in get “ + value + “ “ + i); z = value; } get { System.Console.WriteLine(“in set “ + i); return z; } } } Output In get 30 hi In set hi 30 The this property has a return value, in this case, an int. Also the [] brackets can contain data types other than an int. In this case a string. The string i has a value hi as that is what we passed in the array brackets. You can have two this’s in your class. You have to decide what data type to use in the array brackets. An indexer is very useful when you have a database object and you want to access the data in the fields using a notation [“fieldname”] Indexers follow the same concepts of virtual, new, override etc. a.cs class zzz {
- 155. public static voidMain() { yyy a = new yyy(); a[1] = 10; a[“one”] = 10; a[“hi”,2] = 30; } } class yyy { public int this [ int i] { set { System.Console.WriteLine(“one int “+ i + “ “ + value); } } public int this [ string i] { set { System.Console.WriteLine(“one string “+ i + “ “ + value); } } public int this [ string i, int j] { set { System.Console.WriteLine(“one string and int “+ i + “ “ + j + “ “ + value); } } } Output one int 1 10 one string one 10 one string and int hi 2 30 The signature of an indexer is the number and types of formal parameters. The return value and the names of the parameters do not contribute to the indexers signature. Thus we have overloaded the indexers to take an int, string or a string int combination. Each time a different function gets called. The point to understand is that all the indexers have to return the same data type, in our case int. The same rules that apply to function overloading apply here also. Functions cannot differ only by return values. We are sure that for indexers in the next version, C# should/must make an exception. A property is identified by its name, an indexer by its signature. There is no concept of property overloading in C#. a.cs class zzz { public static void Main() { } } class yyy { public static int this [ int i] { set {
- 156. } } } Compiler Error a.cs(9,19): errorCS0106: The modifier ‘static’ is not valid for this item A property can be both an instance member which is the default or static. An indexer unfortunately can only be an instance member and not static. God alone knows why this discrimination against indexers. Once again no rational reason for the above error. Obviously you cannot create a variable with the same name as that of the parameter passed in the indexer. a.cs class zzz { public static void Main() { xxx a = new xxx(); a[2] = 20; System.Console.WriteLine(a[2]); } } class yyy { public virtual int this [ int i] { get { System.Console.WriteLine(“yyy get “ + i); return 20; } set { System.Console.WriteLine(“yyy set “ + value + “ “ + i); } } } class xxx : yyy { public override int this [ int i] { get { int p = base[i]; System.Console.WriteLine(“xxx get “ + i + “ “ + p); return 200; } set { System.Console.WriteLine(“xxx set “ + value + “ “ + i); base[i] = value; } } } Output xxx set 20 2 yyy set 20 2 yyy get 2 xxx get 2 20 200
- 157. The above exampledeals with calling the indexers of the base class. At times when we are overriding code in the derived class, we would like to call the original indexer in the base class first. The first rule that we have to adhere to is that the indexer in the base class must be declared virtual. In the derived class, we are now declaring it with the modifier override. Same rules as above. In the set accessor, we have to call the original as base[i], where i is the index to the indexer. Also we need to pass it the value to initialize itself. This is stored in the variable value. This a[2] in Main gets replaced by base[2] in the set. In get the reverse takes place. Here we need to place base[i] on the right of the equalto sign, the original get will return a value, in this case 20, which we are storing in a variable p. What we do with p as well as the value from the get is our business. a.cs class yyy { public int this [ byte i , string j] { get { return 10; } set { } } int get_Item(byte i,string j) { return 20 } void set_Item(byte i,string j , int value) { } } Compiler Error a.cs(5,1): error CS0111: Class ‘yyy’ already defines a member called ‘get_Item’ with the same parameter types a.cs(9,1): error CS0111: Class ‘yyy’ already defines a member called ‘set_Item’ with the same parameter types Like a property, an indexer also gets a name change. If people can get their bodies pierced then why cannot a indexer get converted to a series of functions starting with get? For a get, the parameters are the same as we pass to an indexer. It has a return value and the type of the indexer. Also the set has one more added parameter and that is the free variable value. 11 Interfaces and Structures Constructors Revisited A constructor is used to initialize the instance of a class as explained earlier. a.cs public class zzz
- 158. { public static voidMain() { System.Console.WriteLine("in main"); bb a = new bb(); bb b = new bb(10); } } public class aa { public aa() { System.Console.WriteLine("in const aa"); } public aa(int i) { System.Console.WriteLine("in const aa" + i); } } public class bb : aa { public bb() { System.Console.WriteLine("in const bb"); } public bb(int i) { System.Console.WriteLine("in const bb" + i); } } Output in main in const aa in const bb in const aa in const bb10 Class aa is the base class. It consists of two constructors. One that takes no parameters and the other that takes an int as a parameter. Class bb is derived from class aa, i.e. aa is the base class, bb the derived class. When we create an object like bb, the compiler does not execute the code for the constructor but instead asks the constructor which constructor of the base class to execute first. As we haven't stated this, by default, the constructor with no parameters get executed. Remember it is the base class constructor which gets executed first and the derived class constructor specifies which base class constructor to call first. In the second case, even though we are calling the constructor with a parameter, the constructor with no parameters in the base class gets called and not the one with one int as a parameter. a.cs public class zzz { public static void Main() { System.Console.WriteLine("in main"); bb a = new bb(); bb b = new bb(10); } } public class aa {
- 159. public aa() { System.Console.WriteLine("in constaa"); } public aa(int i) { System.Console.WriteLine("in const aa" + i); } } public class bb : aa { public bb() : base() { System.Console.WriteLine("in const bb"); } public bb(int i) : base(i) { System.Console.WriteLine("in const bb" + i); } } Output in main in const aa in const bb in const aa10 in const bb10 If we do not specify which constructor of the base class to call, C# by default calls the constructor with no parameters. Which means that C# rewrites our code . When we write bb(), it gets rewritten as bb() : base(). Base is a reserved word. It means call the constructor of the base class with no parameters. For the second constructor, bb(int i), the line gets rewritten to bb(int i) : base(). We now want to call the constructor with one int and hence we write bb(int i) : base(i). That is why the constructor with one int gets called. We have the option to decide which constructor of the base class we would like to call. a.cs public class zzz { public static void Main() { System.Console.WriteLine("in main"); bb a = new bb(); } } public class aa { public aa() { System.Console.WriteLine("in const aa"); } public aa(int i) { System.Console.WriteLine("in const aa" + i); } } public class bb : aa { public bb() : this(20) { System.Console.WriteLine("in const bb"); } public bb(int i) : base(i)
- 160. { System.Console.WriteLine("in const bb"+ i); } } Output in main in const aa20 in const bb20 in const bb A constructor gets called at the time of creation of the object. At the line, new bb(), the compiler asks the constructor of bb as to which constructor of the base class aa to call. Here he was told that the answer lies with this(20). this, like base, is a reserved word. It means call a constructor of the same class and not the base class. Therefore the compiler now asks the one constructor of the derived class bb which constructor of the base class to call. bb(int i) : base(i) tells the compiler to execute the one int constructor of aa. This is the first constructor that gets called. Then the one int constructor of bb gets called and finally the one who started it all, the no parameter constructor of bb. Thus, two derived class constructors get called instead of one. a.cs public class zzz { public static void Main() { System.Console.WriteLine("in main"); aa a = new aa(); } } public class aa { private aa() { } } Compiler Error a.cs(6,8): error CS0122: 'aa.aa()' is inaccessible due to its protection level When you create a constructor which is private, you cannot create an object that looks like aa. Thus aa should only contain static members. a.cs public class zzz { public static void Main() { } } public class aa { private aa() { } } public class bb : aa { } Compiler Error a.cs(13,14): error CS0122: 'aa.aa()' is inaccessible due to its protection level Nor can any class derive from aa. Thus no one can instantiate an object that looks like aa or derive from it as the constructor has been made private. a.cs
- 161. public class zzz { publicstatic void Main() { System.Console.WriteLine(aa.i); } } public class aa { private aa() { } static public int i = 20; } Output 20 You can however use all the static variables in aa a.cs public class zzz { public static void Main() { } } class yyy { public yyy() { } } class xxx : yyy { public int i; xxx() : base( this.i) { } } Compiler Error a.cs(16,15): error CS0027: Keyword this is not available in the current context Base is called a constructor initializer. When base gets called, the instance or the object has not yet been created. Ergo, this is not available here as this refers to the current object. In the constructor, however, this can be freely used. The values of variables in a class are initialized to their default values as per their data types before the constructor gets called. Thus, in the constructor they have their default values as shown below. a.cs public class zzz { public static void Main() { yyy a = new yyy(); } } class yyy { public int i;
- 162. public bool j; publicyyy() { System.Console.WriteLine(i+" " + j); } } Output 0 False Here as before, the first line of code in the constructor gets executed. a.cs public class zzz { public static void Main() { xxx a = new xxx(); } } class yyy { public int i = 10; public yyy(int j) { System.Console.WriteLine(i); i = j; } } class xxx : yyy { public xxx() : base(100) { System.Console.WriteLine(base.i); } } Output 10 100 Calling the base class constructor is like inserting all the code of the one int constructor i.e. yyy(int j), in the constructor of class xxx. We are also allowed to access members of the base class after the constructor gets called. Also, first the variable i gets initialized to 10 or the default value of int. Then we change it to 100 and in the constructor of xxx, we will see a value of 100. a.cs public class zzz { public static void Main() { xxx a = new xxx(); } } class yyy { public yyy() { abc(); } public virtual void abc() {
- 163. } } class xxx :yyy { public int x = 10; public xxx() { System.Console.WriteLine(x); x = 100; System.Console.WriteLine(x); } public override void abc() { System.Console.WriteLine(x); } } Output 10 10 100 We have already confessed a million times in the past that we have copied ideas from anyone and everyone, specially from the documentation. In this specific case, we wanted to demonstrate that first the variables are initialized. Thus in class xxx, the int x is initialized to 10. Then the base class constructor of yyy gets called. The value of x in class yyy should be 10. But what guarantee can we give you as we are not able to print the value of x in an object of a class derived from the base class. Very simple. We call a virtual function abc from class yyy and override it in class xxx. The abc of class xxx prints the value of x as they belong to the same class and the output is 10. Viola and thank you Mr. Documentation for the above thought and many more such ideas. Once all code in the yyy constructor is executed, the first line in xxx constructor will get executed which will print the value of x as 10. x is then initialized to 100, hence we see 100 as the new value of x, displayed on the screen. a.cs public class zzz { static zzz() { System.Console.WriteLine("zzz"); } public static void Main() { System.Console.WriteLine("main"); new aa(); } } public class aa { public aa() { System.Console.WriteLine("aa"); } static aa() { System.Console.WriteLine("static aa"); } } Output zzz main static aa aa
- 164. and if wecomment new aa() then the resulting output reads as follows. Output zzz main A static constructor gets called before any other constructors. a.cs public class zzz { static zzz() { System.Console.WriteLine("zzz"); } public static void Main() { System.Console.WriteLine("main"); aa.a(); } } public class aa { public aa() { System.Console.WriteLine("aa"); } public static void a() { } static aa() { System.Console.WriteLine("static aa"); } } Output zzz main static aa If you try to access any static member of a class or whenever you instantiate an object, the static constructor gets called. A constructor is not inherited by the derived class. A class is loaded in memory before any instance of the class is created or its static members accessed. A class can only be loaded once and that too before its derived class is loaded. The static constructor is called at the time of loading the class. Like other constructors, a static constructor cannot be explicitly called. a.cs public class zzz { public static void Main() { xxx.pqr(); yyy.abc(); } } class yyy { static yyy() { System.Console.WriteLine("static yyy");
- 165. } public static voidabc() { System.Console.WriteLine("abc yyy"); } } class xxx { static xxx() { System.Console.WriteLine("static xxx"); } public static void pqr() { System.Console.WriteLine("pqr xxx"); } } Output static xxx pqr xxx static yyy abc yyy Do not believe the above results as if you run them on your machine, your mileage may wary. This is because C# does mandate the order of loading of classes and thus the order of execution of the static constructors. On your machine if the yyy constructor gets executed first, do not panic. Blame it on your destiny. a.cs public class zzz { public static void Main() { yyy.abc(); xxx.pqr(); } } class xxx { static xxx() { System.Console.WriteLine("static xxx"); } public static void pqr() { System.Console.WriteLine("pqr xxx"); } } class yyy : xxx { static yyy() { System.Console.WriteLine("static yyy"); } public static void abc() { System.Console.WriteLine("abc yyy"); } } Output static yyy abc yyy
- 166. static xxx pqr xxx Wehave made only one small change in the above program. We have derived the class yyy from xxx. Since the functions in each class are marked static, the program behaves in the same manner as before.The above order of calls remains the same. If an object of the type is created, then the output will change. The reason being that before the object of type yyy is created, xxx must be loaded. Hence the constructors will be called first. a.cs public class zzz { public static int x = yyy.y + 2; static zzz() { System.Console.WriteLine("static zzz " + zzz.x + " " + yyy.y); x = 500; yyy.y = 600; System.Console.WriteLine("static zzz " + zzz.x + " " + yyy.y); } public static void Main() { System.Console.WriteLine("main "+ zzz.x + " " + yyy.y); } } class yyy { static yyy() { System.Console.WriteLine("static yyy " + zzz.x + " " + yyy.y); y = 10; zzz.x = 200; System.Console.WriteLine("static yyy " + zzz.x + " " + yyy.y); } public static int y = zzz.x + 3; } Output static yyy 0 3 static yyy 200 10 static zzz 12 10 static zzz 500 600 main 500 600 Difficult code to understand and follow. C# first tries to load class zzz in memory as it contains the function Main. Unfortunately it realizes that it has to first initialize the variable x before calling the static constructor of zzz. It first initializes x to 0. Now note that this initialization of x to zero is extremely significant for our understanding. To get the new value of x, C# now needs the value of the variable y from the class yyy. Before it can call the static constructor of yyy it must initialize the variable y. It makes sure that y's value is first set to zero. It then computes the value of zzz.x which is zero as stated above. We are yet left hanging in class zzz at the line x = . As zzz.x is zero, the value of y is 0 + 3 i.e. 3. This completes the initialization of all the variables in the class. All this happens first. Thus the static constructor of class yyy shows the value of the variable x as 0 and that of variable y as 3. But hold on, the fun is yet to begin. In the static constructor, we now initialize y to 10 and the x of zzz to 200. The next WriteLine confirms that our initializations actually were carried out. Then we go back to class zzz. Here we come back to the initialization of the static variable x. As yyy.y is now 10 since we changed it in the static constructor of yyy, the value of x is 10 + 2 i.e. 12. This overrides the value of x which we changed to 200 in the static constructor yyy. Now C# calls the static constructor of zzz as it has finished all the variable initializations. Thus the first WriteLine displays 12 and 10. We are now changing both x and y and they display the same values in the constructor and in Main. a.cs public class zzz
- 167. { public static intx = yyy.y + 2; static zzz() { System.Console.WriteLine("static zzz " + zzz.x + " " + yyy.y); x = 500; yyy.y = 600; System.Console.WriteLine("static zzz " + zzz.x + " " + yyy.y); } } class yyy { static yyy() { System.Console.WriteLine("static yyy " + zzz.x + " " + yyy.y); y = 10; zzz.x = 200; System.Console.WriteLine("static yyy " + zzz.x + " " + yyy.y); } public static int y = zzz.x + 3; public static void Main() { System.Console.WriteLine("main "+ zzz.x + " " + yyy.y); } } Output static zzz 2 0 static zzz 500 600 static yyy 500 503 static yyy 200 10 main 200 10 The above program adds a small twist. It bowls what in cricket parlance is called a googly. We simply bring Main from the class zzz to the class yyy. Now C# as usual first starts at the class containing Main which now happens to be yyy and not zzz. Here it has to first initialize all the variables in class yyy. We have only one. It starts by setting y to zero and runs to the class zzz to fetch the value of x. x now become 0 + 2 i.e. 2. Then in the static constructor, we are displaying the relevant values of x and y. In the static constructor of class zzz, we are changing x and y to 500 and 600 respectively and displaying the values. When we move back to class yyy, however, y gets a new value of 503 as x is 500. y loses its value of 600 that was initialized in zzz, hence you see 500 and 503. The rest remains the same as explained in the earlier example. Interfaces An interface is simply a collection of function prototypes. Like we derive a class from another, so also we could derive from an interface. a.cs class zzz { public static void Main() { } } interface ddd { void a1(); void a2(); } class yyy : ddd {
- 168. } Compiler Error a.cs(12,7): errorCS0535: 'yyy' does not implement interface member 'ddd.a1()' a.cs(12,7): error CS0535: 'yyy' does not implement interface member 'ddd.a2()' We have just created an interface called ddd by using a new keyword interface in place of a class. Our interface ddd has two function prototypes, a1 and a2. We can derive from our interface ddd like we derived from a class. The difference is that an interface has no code, only function prototypes. Whenever we derive from an interface, we have to implement the code or body of the function. A class gives you lots of free code, an interface does not. The error is generated as we have not given the code for a1 and a2 in yyy a.cs class zzz { public static void Main() { yyy a = new yyy(); a.a1(); ddd d = new yyy(); d.a2(); } } interface ddd { void a1(); void a2(); } class yyy : ddd { public void a1() { System.Console.WriteLine("yyy a1"); } public void a2() { System.Console.WriteLine("yyy a2"); } } Output yyy a1 yyy a2 We get no errors because we have now implemented the code of a1 and a2. Looks wise we do no know whether ddd is a class or an interface as the syntax at the time of derivation is the same. d is an object that looks like an interface which is syntactically correct. d can be equated to a yyy as a yyy is a yyy + a ddd. We can, by only using d, call members of a ddd. a.cs class zzz { public static void Main() { aaa a ; a = new aaa(); } } interface aaa { } Compiler Error a.cs(6,5): error CS0144: Cannot create an instance of the abstract class or interface 'aaa'
- 169. Even though aninterface aaa is empty, we cannot write the keyword new in front of it. An interface contains no code and thus cannot be instantiated. However we are allowed to declare objects that look like an interface. Therefore, in this case, the line aaa a, does not flag an error. a.cs class zzz { public static void Main() { } } interface aaa { void a1() { } } Compiler Error a.cs(9,6): error CS0531: 'aaa.a1()': interface members cannot have a definition Reiterating, an interface can only contain function prototypes, no code at all. The functions cannot have a definition. a.cs class zzz { public static void Main() { } } class xxx { } class vvv { } class yyy : xxx,vvv { } Compiler Error a.cs(13,17): error CS0527: 'vvv' : type in interface list is not an interface C# does not support multiple inheritance. We can derive from a single class only at one point in time. a.cs class zzz { public static void Main() { yyy a = new yyy(); ddd d = new yyy(); d.a1(); d.a2(); a.a1(); a.a2(); } } interface ddd { void a1(); void a2();
- 170. } class yyy :ddd { public void ddd.a1() { System.Console.WriteLine("a1"); } public void a2() { System.Console.WriteLine("a2"); } } Compiler Error a.cs(19,13): error CS0106: The modifier 'public' is not valid for this item You are not allowed to use the modifier public for a function which has qualified its name with that of the interface. a.cs class zzz { public static void Main() { } } interface aaa { public void a1(); } Compiler Error a.cs(9,13): error CS0106: The modifier 'public' is not valid for this item Interface members are public by default. The access modifiers are not allowed here. All the other access modifier rules remain the same as that from classes. The rules in classes stated that the base class must be at least as accessible as the derived class. Replace the word class with interface and you will not be sorry. a.cs class zzz { public static void Main() { yyy a = new yyy(); ddd d = new yyy(); d.a1(); d.a2(); a.a1(); a.a2(); } } interface ddd { void a1(); void a2(); } class yyy : ddd { void ddd.a1() { System.Console.WriteLine("a1"); } public void a2() {
- 171. System.Console.WriteLine("a2"); } } Compiler Error a.cs(8,1): errorCS0117: 'yyy' does not contain a definition for 'a1' The reason we get an error is because we created the function a1 in yyy as ddd.a1 and not a1. By doing this, we were telling C# that only objects that look like ddd are allowed access to a1. Even an object that looks like yyy is not allowed to access a1. Comment out line number 8 i.e. a.a1 and all works fine as follows Output a1 a2 a2 a.cs class zzz { public static void Main() { yyy a = new yyy(); ddd d = new yyy(); eee e = new yyy(); a.a1(); a.a2(); d.a1(); e.a2(); } } interface ddd { void a1(); } interface eee { void a2(); } class yyy : ddd , eee { public void a1() { System.Console.WriteLine("a1"); } public void a2() { System.Console.WriteLine("a2"); } } Output a1 a2 a1 a2 Here we are doing something that just cannot be done with classes. We are deriving from two interfaces ddd and eee at the same time. Each has one function prototype a1 and a2 respectively. Using a which looks like yyy we can call both a1 and a2 but with d that looks like ddd we can only call a1. Similarly, with e only a2 can be called. a.cs
- 172. class zzz { public staticvoid Main() { yyy a = new yyy(); ddd d = new yyy(); eee e = new yyy(); a.a1(); d.a1(); e.a1(); } } interface ddd { void a1(); } interface eee { void a1(); } class yyy : ddd , eee { public void a1() { System.Console.WriteLine("a1"); } } Output a1 a1 a1 The two interfaces share the same function name a1. We do not get an error but yet things do not seem right. Both d and e call the same a1 and there is only one implementation of the function a1. We would like to have two a1's but we cannot have the same function defined twice in a class. a.cs class zzz { public static void Main() { ddd d = new yyy(); eee e = new yyy(); d.a1(); e.a1(); } } interface ddd { void a1(); } interface eee { void a1(); } class yyy : ddd , eee { void ddd.a1() { System.Console.WriteLine("ddd a1"); }
- 173. void eee.a1() { System.Console.WriteLine("eee a1"); } } Output ddda1 eee a1 We did what we had explained earlier. We prefaced the name of the function with the name of the interface. Then we removed the modifier public and the object a which looked like yyy. Now each interface has its own copy of a1 to be called. ddd.a1 is called the explicit interface member name. a.cs class zzz { public static void Main() { yyy a = new yyy(); a.a1(); } } interface ddd { void a1(); } interface eee { void a1(); } class yyy : ddd , eee { void ddd.a1() { System.Console.WriteLine("ddd a1"); } void eee.a1() { System.Console.WriteLine("eee a1"); } } Compiler Error a.cs(6,1): error CS0117: 'yyy' does not contain a definition for 'a1' The reason we removed a was that once we have an explicit interface member, we cannot access it through the class, it is done only through the interface. This make a lot of sense as there are two functions of the same name, and C# does not know which one it should call. By prefacing the functions with the names of the interface, we are making them part of the interface and not the class. In a sense they are private to the class. As there is no function by the name of a1 in class yyy, we see the above error. a.cs class zzz { public static void Main() { } } interface ddd { void a1();
- 174. } interface eee { void a1(); } classyyy : ddd { void ddd.a1() { } void eee.a1() { } } Compiler Error a.cs(21,6): error CS0540: 'yyy.eee.a1()': containing class does not implement interface 'eee' We get an error as class yyy is derived only from interface ddd. Thus we cannot use eee.a1() as interface eee is not a base interface for class yyy. a.cs class zzz { public static void Main() { } } interface ddd { void a1(); } class yyy : ddd { void ddd.a1() { } } class xxx : yyy { void ddd.a1() { } } Compiler Error a.cs(19,6): error CS0540: 'xxx.ddd.a1()': containing class does not implement interface 'ddd' The key concept earlier was that we could only use the name of an interface explicitly if it was categorically stated as a base class. Anything indirect would not do. Here class yyy is derived from interface ddd and thus we can use the form ddd.a1. However even though class xxx is derived from class yyy and thus also from interface ddd, we are not allowed to write ddd.a1 as interface ddd is not explicitly stated in the derivation list of class xxx. An interface defines a contract and can only contain four entities viz methods, properties, events and indexers. An interface thus cannot contain constants, fields, operators, constructors, destructors, static constructors, or types. Also an interface cannot contain static members of any kind. The modifiers abstract, public, protected, internal, private, virtual, override are disallowed as they make no sense in this context. a.cs class zzz { public static void Main() {
- 175. } } public delegate voidd1(); public interface aaa { void a1(string s); int a2 { get; set ; } event d1 ddd; string this[int i] { get; set; } } The above example demonstrates the four entities an interface can contain. Anything else will flag an error. a.cs class zzz { public static void Main() { ccc c = new ccc(); aaa a = c; bbb b = c; c.a1();c.a2(); a.a1(); b.a1();b.a2(); } } interface aaa { void a1(); } interface bbb : aaa { void a2(); } class ccc : bbb { public void a1() { System.Console.WriteLine("a1"); } public void a2() { System.Console.WriteLine("a2"); } } Output a1 a2 a1 a1 a2 An interface can also, like a class, inherit from one or more interfaces. In this case bbb as an interface, can inherit from aaa. The class ccc inherits from interface bbb and and thus interface aaa has to implement functions a1 and a2. The object c not only looks like a ccc but also looks like an aaa and bbb. Thus equating them does not give an error. The reverse, however, is not true. It will obviously flag an error. However a can only access functions from the interface it belongs to, in this case a1 and b can access a1 and a2. b cannot access
- 176. members from classccc even though a, b, and c have the same values. Whenever an interface derives from another it is called an explicit base interface. Like classes, circular definitions are not permitted. In fact nowhere in the C# programming language are we permitted to have circular definitions. An interface also creates a new type and as in classes, methods must have their own unique signatures. Properties and methods cannot have similar names. a.cs class zzz { public static void Main() { } } interface aaa { void a1(); } interface bbb : aaa { void a1(); } Compiler Warning a.cs(13,6): warning CS0108: The keyword new is required on 'bbb.a1()' because it hides inherited member 'aaa.a1()' Like classes if interfaces derive from each other, there is a possibility that they may contain the same function signatures. Thus we get a warning which can be removed by adding the keyword new as follows. interface bbb : aaa { new void a1(); } The point to be stressed here is that the keyword new really does not do much, other than remove the warning. This is because the implementation of the interface is done in the class. There, we will have only one implementation of function a1 and not two, one for aaa and the other for bbb. At one level, from the point of view of the class which derives from interface bbb, it will see only one function in it, not two. All that we are doing is hiding the base interface member. a.cs class zzz { public static void Main() { } void abc(ccc c) { c.aa(1); ((bbb)c).aa(1); ((aaa)c).aa(1); c.aa = 2; ((aaa)c).aa = 3; ((bbb)c).aa = 3; } } interface aaa { int aa {
- 177. get; set; } } interface bbb { voidaa(int i); } interface ccc : aaa , bbb { } Compiler Error a.cs(8,1): error CS0229: Ambiguity between 'aaa.aa' and 'bbb.aa(int)' a.cs(10,10): error CS0118: 'aaa.aa' denotes a 'property' where a 'method' was expected a.cs(11,1): error CS0229: Ambiguity between 'aaa.aa' and 'bbb.aa(int)' a.cs(13,10): error CS0654: Method 'bbb.aa(int)' referenced without parentheses Interface aaa has only one member, a property called aa, whereas interface bbb has one function called aa. The interface ccc does not give us an error in spite of the fact that we have a property and a function with the same name. In the function abc, we are passing c, an object that looks like ccc. The line c.aa(1) gives us an error as we have a property and a function called aa. C# gets confused whether it is the property or the function we are referring to. In our humble opinion, we could only be referring to the function as per the syntax, but you don't argue with a compiler. In the second case c.aa = 2 also flags an error due to the name confusion. In this case also, we could only be referring to a property. The only way out is to cast. The second cast in each case gives an error as the syntax for calling a function and property is different. Normally, to cast, we need to give two sets of brackets. A cast incurs no run time costs i.e., it does not slow down the program. All that a cast does in the above case is lowers the pointer from a ccc to aaa or bbb at compile time. In an earlier example, we spoke of functions from two interfaces having similar names. The same rules stated there apply here also. a.cs class zzz { public static void Main() { } void abc(ccc c) { c.aa(1); c.aa((byte)1); ((aaa)c).aa(1); ((bbb)c).aa(1); } } interface aaa { void aa(byte i); } interface bbb { void aa(short i); } interface ccc : aaa , bbb { } Compiler Error a.cs(8,1): error CS0121: The call is ambiguous between the following methods or properties: 'bbb.aa(short)' and 'aaa.aa(byte)'
- 178. a.cs(9,1): error CS0121:The call is ambiguous between the following methods or properties: 'bbb.aa(short)' and 'aaa.aa(byte)' We have a similar problem again. c.aa(1) does not know which function aa to call. C# could convert the 1 an int, either to a short or a byte. Thus the ambiguity. Even if we cast the 1 to a byte, for some reason, C# yet gives us an error. The only way out is like what we did earlier, explicitly cast your way out of trouble. When we cast, we are restricting ourselves to only one method and thus no ambiguity. a.cs class zzz { public static void Main() { } } interface aaa { void aa(); } interface bbb : aaa { new void aa(); } interface ccc : aaa { void cc(); } interface ddd : bbb , ccc { } class eee : ddd { } Compiler Error a.cs(22,7): error CS0535: 'eee' does not implement interface member 'bbb.aa()' a.cs(22,7): error CS0535: 'eee' does not implement interface member 'ccc.cc()' a.cs(22,7): error CS0535: 'eee' does not implement interface member 'aaa.aa()' The interface aaa has one function aa. The interface bbb is derived from aaa and also has one function called aa. The keyword new informs the C# compiler that, it has hidden or has nothing to do with the function aa in interface aaa. Remember in interfaces, we cannot write any code. The interface ccc also derives from aaa but does not have a function called aa. Then we are creating another interface ddd which derives from both bbb and ccc. The class eee is then derived from interface ddd. We get three errors as we have to implement three functions in class eee. The interface ccc brings in two functions. One cc and the other aa from interface aa. The interface bbb has only one function, aa which is different from the aa which is present in interface aaa, thanks to the keyword new which is optional. a.cs class zzz { public static void Main() { aaa a = new eee(); a.aa(); bbb b = new eee(); b.aa(); ccc c = new eee(); c.aa(); ddd d = new eee(); d.aa();
- 179. eee e =new eee(); //e.aa(); ((aaa)d).aa(); ((bbb)d).aa(); ((ccc)d).aa(); ((ddd)d).aa(); } } interface aaa { void aa(); } interface bbb : aaa { new void aa(); } interface ccc : aaa { void cc(); } interface ddd : bbb , ccc { } class eee : ddd { void aaa.aa() { System.Console.WriteLine("aa aaa"); } void bbb.aa() { System.Console.WriteLine("aa bbb"); } public void cc() { System.Console.WriteLine("cc"); } } Output aa aaa aa bbb aa aaa aa bbb aa aaa aa bbb aa aaa aa bbb With or without the keyword new, the output remains the same. By adding the keyword new, all that we have achieved is removal of the warning. Lets explain the above program step by step. We have implemented the three functions that class eee needed. We could have implemented only one aa, and then in every case that one aa, would have been called. In this case we need two aa functions. One for the interface aaa and the other for the interface bbb. As we need a separate function aa for each interface we will land up having two of them. Thus we need to preface them with the name of the interface. We are creating objects that look like all our interfaces and initializing them to an object that looks like class eee. The object a that looks like interface aaa, will obviously call the aa of interface aaa. The object b that looks like interface bbb will call the aa of interface aaa implemented in class eee. The object c, that looks like ccc is derived from interface aaa and thus it will call the function aa of interface aaa. All of this is obvious. Now d looks like ddd which is derived from ccc and bbb. The reason it will call the aa of bbb and not of aaa is because we have asked it to hide the function aa of interface aaa. Once a function is hidden, it will remain hidden forever
- 180. in all theaccess paths. Thus function aa of interface bbb hides the function aa of interface aaa even in interface ccc. The casting gives us the same answer as above. In the first case, cast d, a ddd looks alike to an aaa. Thus the aa from aaa gets called. In the second case it calls the aa of bbb. Just like before, when we cast c to a ccc, it calls the aa form aaa . Now we cast c to a ddd, it can call either the function aa from bbb or function aa from aaa. In this case it chooses to call the aa from bbb as its aa hides the aa of interface aaa. To sum up, if a function is hidden from one access path, it is hidden from all. A little later in this chapter, we will show you that structures can also implement from an interface.This process of implementing all the functions is called interface mapping. a.cs class zzz { public static void Main() { } } interface aaa { void a1(); } interface bbb : aaa { new int a1 { set; } } class ccc : bbb { void aaa.a1() { } int bbb.a1 { set { } } } class ddd: bbb { public void a1() {} int bbb.a1 { set { } } } class eee: bbb { void aaa.a1() { } public int a1 { set {
- 181. } } } The interface aaahas one function a1 which clashes with the name of the property in interface bbb. We have three ways to create a class which is derived from bbb. In the first case ccc, we are explicitly qualifying with the name of interface. In the second case of ddd, we are qualifying only the property and not the function. In the third case i.e. class eee, we do the reverse. Thus one of the a1or both must be qualified or else it will result in an error. a.cs class zzz { public static void Main() { } } interface aaa { void a1(); } interface bbb : aaa { void a2(); } interface ccc : aaa { void a3(); } class ddd : aaa,bbb,ccc { void bbb.a1() { } public void a2() { } public void a3() { } } Compiler Error a.cs(21,6): error CS0539: 'bbb.a1' in explicit interface declaration is not a member of interface a.cs(19,7): error CS0535: 'ddd' does not implement interface member 'aaa.a1()' Both interfaces bbb and ccc derive from interface aaa. Class ddd derives from three interfaces aaa and bbb and ccc. In class ddd, we have only 3 functions, a1, a2 and a3 that come from aaa, bbb and ccc respectively. Even though interface bbb derives from aaa, it does not carry a1 into class ccc. So, we cannot write bbb.a1 in class ccc. Hence, we have only one a1 from interface aaa in class ccc and none from interfaces bbb and ccc. This proves that C# forgets the interfaces bbb and ccc are derived from. It also means that bbb and ccc cannot have their own implementations of a1. a.cs class zzz { public static void Main() { aaa a = new ccc(); a.a1(); }
- 182. } interface aaa { void a1(); } classbbb { public void a1() { System.Console.WriteLine("bbb a1"); } } class ccc : bbb , aaa { } Output bbb a1 The class ccc derives from bbb and aaa. The function a1 from interface aaa is implemented by class bbb and not by ccc. No one cares as long as there is a function a1 somewhere. The function a1, in this case, can be in the actual class or in the base class. a.cs class zzz { public static void Main() { bbb b = new ccc(); b.a1(); ccc c = new ccc(); c.a1(); aaa a = new ccc(); a.a1(); aaa aa = new bbb(); aa.a1(); } } interface aaa { void a1(); } class bbb : aaa { public void a1() { System.Console.WriteLine("bbb a1"); } } class ccc : bbb { new public void a1() { System.Console.WriteLine("ccc a1"); } } Output bbb a1 ccc a1 bbb a1 bbb a1
- 183. In this case,class ccc is derived from class bbb, which in turn is derived from interface aaa. Both the classes implement the function a1. The question on our minds is from which class will the function a1 be called. Both b and c are initialized to an object that looks like ccc. b.a1() will obviously call the function a1 from bbb and c.a1() will call it from ccc. Saying the same thing over and over again like a malfunctioning record disk is trying but essential. Objects a and aa look like aaa but are initialized to objects that look like ccc and bbb respectively. It does not seem to make any difference as each time the function a1 gets called from the base class bbb and not from the derived class ccc or the class that we have initialized the object to. This means that the derived class cannot alter the interface mappings it receives from the base class. The mapping of function a1 is to the class bbb as bbb was derived from the interface. Class ccc cannot change this fact. a.cs class zzz { public static void Main() { bbb b = new ccc(); b.a1(); bbb b1 = new bbb(); b1.a1(); ccc c = new ccc(); c.a1(); aaa a = new ccc(); a.a1(); aaa aa = new bbb(); aa.a1(); } } interface aaa { void a1(); } class bbb : aaa { public virtual void a1() { System.Console.WriteLine("bbb a1"); } } class ccc : bbb { public override void a1() { System.Console.WriteLine("ccc a1"); } } Output ccc a1 bbb a1 ccc a1 ccc a1 bbb a1 We have made two changes in our program. We made the function a1 virtual in the class bbb and also, in the class ccc, we declared a1 with the override modifier. Whenever we declare a function to be virtual, the derived classes will and can override it. The modifier override has the opposite meaning over new. It means do not create a new function but override the base class definition.
- 184. Objects b andb1 both look like bbb but are initialized to objects that look like ccc and bbb respectively. When we write b.a1(), C# first looks into the class bbb as that is the data type of b. Here the function a1 is marked virtual, so C# now asks ‘what was b initialized by’? As the answer is an object that looks like ccc, C# now looks at the class ccc for the function a1. As it is marked override, C# will execute the function a1 from class ccc and not bbb. If the override was missing, the default is new, and the function will be called from the data type of the object i.e. bbb. As object b1 has been initialized by an object that looks like bbb, C# will call a1 from class bbb only. The object c follows the earlier specified rules. As for objects a and aa, both look like interface aaa which has no code for the function aaa. Here the object used to initialize will decide the class a1 will be called from. This is due to the explanation above as a1 is marked virtual in class bbb and override in class ccc. a.cs class zzz { public static void Main() { } } interface ppp { void a1(); } interface aaa { void a1(); } class bbb : aaa , ppp { public virtual void aaa.a1() { System.Console.WriteLine("bbb a1"); } } class ccc : bbb { public override void a1() { System.Console.WriteLine("ccc a1"); } } Compiler Error a.cs(17,21): error CS0106: The modifier 'public' is not valid for this item a.cs(17,21): error CS0106: The modifier 'virtual' is not valid for this item Lets make matters worse. We have an interface ppp which also brings in a function called a1. The class bbb also derives from ppp. Thus we have two a1 functions and we have to explicitly qualify them. Unfortunately we cannot add the qualifier virtual here. The only way out is as follows. a.cs class zzz { public static void Main() { aaa a = new ccc(); a.a1(); aaa aa = new bbb(); aa.a1(); } } interface ppp
- 185. { void a1(); } interface aaa { voida1(); } class bbb : aaa , ppp { void aaa.a1() { aa1(); } void ppp.a1() { } public virtual void aa1() { System.Console.WriteLine("aa1"); } } class ccc : bbb { public override void aa1() { System.Console.WriteLine("aa1 ccc"); } } Output aa1 ccc aa1 What we have done here is that in the a1 function of aaa we have called the function aa1. So in other words, a1 simply calls aa1. We have made aa1 virtual such that each time aa1 is called, the question asked will be - which object was used at the time of initialization? Depending upon the type of object, the relevant function aa1 would be executed. Thus we get the same answer as earlier. If we have to reimplement an interface, you can look at the example earlier. C# forgets about the initial derivation. a.cs class zzz { public static void Main() { } } interface aaa { void a1(); void a2(); } abstract class bbb : aaa { } Compiler Error a.cs(12,16): error CS0535: 'bbb' does not implement interface member 'aaa.a1()' a.cs(12,16): error CS0535: 'bbb' does not implement interface member 'aaa.a2()' Whenever we have a abstract class deriving from an interface, the class must implement all the methods of the interface as follows.
- 186. a.cs class zzz { public staticvoid Main() { } } interface aaa { void a1(); void a2(); } abstract class bbb : aaa { public void a1() { } public abstract void a2(); } If the abstract class does not want to implement all the members of the interface, it has to mark the functions as abstract and public. The class deriving from bbb will now have to supply the code of the class bbb Structures a.cs class zzz { public static void Main() { xxx x = new xxx(10); System.Console.WriteLine(x.i); x.abc(); } } struct xxx { public int i; public xxx( int j) { i = j; } public void abc() { System.Console.WriteLine("abc"); } } Output 10 abc A structure or struct to be precise is similar to a class in many ways. A struct can contain fields, methods etc. If a struct and class were 100 per cent similar, then why have both? We will now explain the differences between a struct and a class. Unless otherwise stated, they share the same features. a.cs class zzz { public static void Main()
- 187. { } } struct xxx { public inti = 10; } Compiler Error a.cs(9,12): error CS0573: 'xxx.i': cannot have instance field initializers in structs In a class we are allowed to create a field/variable and initialize it at the same time. A structure cannot contain such initializations. Thus these fields must be initialized either through functions or by using the object itself. Fields cannot be given initial values at the time of creation. a.cs class zzz { public static void Main() { xxx a = new xxx(); } } struct xxx { public int i; public xxx() { } } Compiler Error a.cs(11,8): error CS0568: Structs cannot contain explicit parameterless constructors A class can contain many constructors. And even if we do not have any, C# supplies a free constructor with no parameters. It's very different with structures. You cannot have a constructor with no parameters. If you do, you will get the above error. Thus C# lets you have as many constructors as you like but none without parameters. a.cs class zzz { public static void Main() { xxx a = new xxx(10); } } struct xxx { public int i; public xxx(int p) { } } Compiler Error a.cs(10,8): error CS0171: Field 'xxx.i' must be fully assigned before control leaves the constructor A small problem here. As mentioned earlier we cannot initialize a field like we do in a class. Hence C# insists on constructors that accept parameters to initialize fields; at the end of the constructor if you have not initialized the fields in the structure, we get the above error. Thus C# actually reads our code and makes sure that the constructor initializes all the fields in the structure. a.cs class zzz { public static void Main()
- 188. { xxx a =new xxx(10); } } struct xxx { public int i,j; public xxx(int p) { i = p; } } Compiler Error a.cs(11,8): error CS0171: Field 'xxx.j' must be fully assigned before control leaves the constructor The keyword shown in all the above errors is fields. a.cs class zzz { public static void Main() { xxx a = new xxx(10); } } struct xxx { public int i,j; public xxx(int p) { i = p; j = 0; } } We get no error as the structure has two fields and we have now initialized both of them. a.cs class zzz { public static void Main() { xxx a = new xxx(); System.Console.WriteLine(a.i); } } struct xxx { public int i; } Compiler Warning a.cs(11,12): warning CS0649: Field 'xxx.i' is never assigned to, and will always have its default value 0 Output 0 If we do not have any constructors, we get no error. As we have not initialized the field i, in the constructor, we have none. C# gives you a warning and for the sake of your sanity, initializes i to 0. a.cs class zzz {
- 189. public static voidMain() { xxx a = new xxx(); System.Console.WriteLine(a.i); } } struct xxx { public int i; } struct yyy : xxx { } Compiler Error a.cs(13,14): error CS0527: 'xxx' : type in interface list is not an interface A structure yyy cannot be derived from another structure. A useful feature of a class was the fact that we could share/use code from other classes. A structure does not support the concept of inheritance. Had xxx been a class we would have received the same error. a.cs class zzz { public static void Main() { xxx a = new yyy(); } } interface xxx { } struct yyy : xxx { } The previous error message gave us a hint . It expected xxx to be a interface. Thus we can derive a structure from an interface. As xxx has no members we do not have to implement any code in our structure. a.cs class zzz { public static void Main() { xxx a = new yyy(); } } struct yyy { } class xxx : yyy { } Compiler Error a.cs(11,7): error CS0509: 'xxx' : cannot inherit from sealed class 'yyy' As a structure could not inherit from a class, the reverse is also true. xxx cannot inherit from a structure yyy. Internally a structure behaves as a sealed class as the error message reads. For those who came in late, a sealed class cannot be used a base class. a.cs
- 190. class zzz { public staticvoid Main() { } } sealed struct yyy { } Compiler Error a.cs(7,15): error CS0106: The modifier 'sealed' is not valid for this item You cannot declare a structure as sealed as they are by definition sealed. If you add the word sealed as we have done, C# complains about the fact that we are repeating the same modifier again and nobody likes repetition. a.cs class zzz { public static void Main() { } } abstract struct yyy { } Compiler Error a.cs(7,17): error CS0106: The modifier 'abstract' is not valid for this item As C# does not like repetition, we have to bow to its wishes. But you do not have an option. We are free to repeat ourselves with you as many times as we like. So let us indulge ourselves. A structure as mentioned earlier cannot be used as a base class i.e. be used in a derivation. An abstract modifier, means an incomplete entity which cannot be used directly but only as a base class. It makes no sense for a structure to use the modifier abstract. In the same vein, function in a structure cannot be marked abstract or virtual as these are concepts, which apply to derived classes only. a.cs class zzz { public static void Main() { } } protected struct xxx { } Compiler Error a.cs(7,1): error CS1527: Namespace elements cannot be explicitly declared as private or protected A structure cannot be derived from and the modifier protected deals with the visibility of members from derived classes. Replacing protected with private results in the same error. a.cs class zzz { public static void Main() { yyy a = new yyy(); System.Console.WriteLine(a.ToString());
- 191. } } struct yyy { } Output yyy However everystructure like a class is implicitly derived from the class Object and thus has the functions Object contains. In this case ToString returns the name of the derived class. a.cs class zzz { public static void Main() { yyy a = new yyy(); System.Console.WriteLine(a.ToString()); } } struct yyy : System.Object { } Compiler Error a.cs(9,14): error CS0527: 'System.Object' : type in interface list is not an interface How do we explain the above error? We just mentioned earlier that as a structure inherits from the class Object and we called a function called ToString from the class Object. However we cannot derive from Object explicitly as the compiler insists on the name of an interface and Object is a name of a class. These little quirks make learning C# difficult. If you replace the word struct with class in the above example, you will not get an error. A computer has lots of memory. We logically divide this memory into two, the stack and the heap. Memory denoted as the stack lives as long as the function lives and heap memory lives as long as your program runs. Objects, instances of a class are allocated on the heap. They will live for a long time. They are also called reference objects. Structures are allocated on the stack by new and live for a shorter duration. We call them value objects. This means that a structure directly represents the data where as a object contains a reference to the data. Structures thus offer a more efficient access to variables than classes. Int, bool and all the other basic data types are implemented as structures. a.cs class zzz { public static void Main() { xxx x,y; yyy a,b; x = new xxx(1); y = new xxx(2); a = new yyy(1); b = new yyy(2); System.Console.WriteLine(x.i + " " + y.i + " " + a.i + " " + b.i); x = y; a = b; System.Console.WriteLine(x.i + " " + y.i + " " + a.i + " " + b.i); x.i = 100; a.i = 200; System.Console.WriteLine(x.i + " " + y.i + " " + a.i + " " + b.i); } } class yyy {
- 192. public int i; publicyyy(int j) { i = j; } } struct xxx { public int i; public xxx(int j) { i = j; } } Output 1 2 1 2 2 2 2 2 100 2 200 200 We have created two objects a and b which look like a class yyy and two structures x and y which look like xxx. In the constructors of each, we are initializing the field i to 1 and 2 respectively. Then we are equating the object a to b and the structure x to y. This will set the i of x to 2 from 1 and also the i of a to 2. Thus the second WriteLine displays all two's. We are now changing the i of x, which is a structure to 100 and we realize that the i of y, the structure it was initialized to remains the same. Thus x.i is 100 and y.i remains at 2. For classes it does not work the same way. Changing a.i to 200, somehow changes the i of b which a was initialized to, also to 200. This could only be possible if when we equate structures, each has its own copy of the data. When we equate objects, no copying takes place and now object a refers to object b in memory. Both a and b represent the same object. Changing a changes b. A little later, we dwell on this concept further. The same rules of passing parameters to a function i.e. out and ref that apply to an int also apply to a structure as int is also a structure internally. a.cs class zzz { public static void Main() { xxx x; yyy a; x=null; a=null; } } class yyy { } struct xxx { } Compiler Error a.cs(7,3): error CS0037: Cannot convert null to 'xxx' because it is a value type In computer programming, a large number of times we do not know the value of a variable at a point in time. These variables are given a value of null. Null can be only applied to objects of reference types and not value types. a.cs class zzz
- 193. { public static voidMain() { } } struct xxx { public ~xxx() { } } Compiler Error a.cs(9,9): error CS0575: Only class types can contain destructors After a long time, we get an error message that is readable. Structures are created on the stack and die when we reach the closing brace. There is thus no memory management to be done from the point of view of the programmer. The structure life cycle is very deterministic and hence no destructors are permitted. 12 Operator Overloading Let’s explain what operator overloading is all about with an example of a class that represents a date. Would it not be great if we could subtract two date objects and be returned an int representing the number of days elapsing between the two dates. We would like to use the good old subtraction operator – like we do when subtracting numbers. Also we would like the > operator to compare two date objects and tell us which one is larger. The + operator could add a number to a date resulting in a new date object. Thus, operator overloading lets us redefine the existing operators so that they work with classes/objects we create like yyy. We have not yet instructed C# on how to use the trusty old + operator with two yyy objects. Though C# knows how to use the + to add two numbers, it does not know how to add two yyy’s. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); yyy c; c = a + b ; } } public class yyy { public int i; public yyy( int j) { i = j; } }
- 194. Compiler Error a.cs(8,5): errorCS0019: Operator ‘+’ cannot be applied to operands of type ‘yyy’ and ‘yyy’ We have created a simple class yyy which has one instance variable i which will distinguish different instances of yyy from each other. The constructor with an int as a parameter initializes i. We have tried to add two objects that look like yyy which does not go down well with C# and it objects to it. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); yyy c; c = a + b ; } } public class yyy { public int i; public yyy( int j) { i = j; } public yyy operator + ( yyy x , yyy y) { } } Compiler Error a.cs(18,12): error CS0558: User-defined operators ‘yyy.operator +(yyy, yyy)’ must be declared static and public Error messages can be at times helpful. Our operator + is public but not static. C# demands that all operator overloads be static. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); yyy c; c = a + b ; System.Console.WriteLine(c.i); } } public class yyy { public int i; public yyy( int j) { i = j; } public static yyy operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); yyy z = new yyy(x.i+y.i); return z; } }
- 195. Output operator + 105 15 The word operator as the name of a function, is legal and the only way to overload operators. We follow this word with the operator we want to overload and then the parameters we will call the operator with. + is a binary operator and will need two yyy’s, one on its left and the other on its right. Then at beginning, we give the return value of the operator. In our case we want a + to add two yyy’s and return a third yyy whose i will be the sum of the individual i’s. Thus a+b will call the operator + with x being equal to a and y to b. Thus x.i will have a value 10 and y.i, 5. We are creating a new object z and in the constructor passing 15 i.e. 10 + 5. Thus the i of z will be 15 which is being returned. a + b will now be replaced by the object whose i has a value 15 and c will be equal to this object. Thus c.i will be equal to 15. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); yyy c; c = a + b ; System.Console.WriteLine(c.i); } } public class yyy { public int i; public yyy( int j) { i = j; } public static yyy operator = ( yyy x , yyy y) { } public static yyy operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); yyy z = new yyy(x.i+y.i); return z; } } Compiler Error a.cs(19,28): error CS1020: Overloadable binary operator expected The error message is telling us that we cannot overload the assignment operator =. Every class gets a free assignment operator which does a bitwise copy of the variables of the object from the left to the right. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); yyy c = new yyy(2); yyy d; d = a + b + c ; System.Console.WriteLine(d.i);
- 196. } } public class yyy { publicint i; public yyy( int j) { i = j; } public static yyy operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); yyy z = new yyy(x.i+y.i); return z; } } Output Operator + 10 5 Operator + 15 2 17 The only change is d = a + b + c. C# gets easily confused with complex statements so it does not read all of it. It sees two operators on the same line. In this case, the same plus. An internal rule tells it to read the plus left to right i.e. it will only see a + b. It will call the operator + with x.i as 10 and y.i as 5 because a’s i is 10 and b’s i is 5. This will create a temporary object like yyy whose i is 15, lets call it zz. The object z is very different from zz. C# then evaluates zz + c. Thus x.i will display 15 and y.i will have the value of c.i i.e. 2. To support multiple invocations of the operator on a single line, the code does not change. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); int d = a + b ; System.Console.WriteLine(d); } } public class yyy { public int i; public yyy( int j) { i = j; } public static int operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); return x.i + y.i; } } Output operator + 10 5 15 C# does not and will never understand what addition of two yyy’s is all about. It is in your hands to decide what the code accomplishes. You decide whether the overloaded + returns a yyy or an int object. The class yyy is your creation not C#’s. Hence you decide what addition means in the context of a yyy class. In this case, we are returning an int unlike earlier where we returned a yyy.
- 197. a.cs public class zzz { publicstatic void Main() { yyy a = new yyy(10); yyy b = new yyy(5); int d = a + b ; System.Console.WriteLine(d); } } public class yyy { public int i; public yyy( int j) { i = j; } public static int operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); return x.i + y.i; } public static yyy operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); yyy z = new yyy(x.i+y.i); return z; } } Compiler Error a.cs(23,19): error CS0111: Class ‘yyy’ already defines a member called ‘ op_Addition ‘ with the same parameter types You cannot have two operator + overloads which only differ in return types. Also the error messages change the name of the operator from + to op_Addition. When we overloaded functions, the return type was not considered part of the function signature. The same applies for operators. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); yyy d = a + b + 20; System.Console.WriteLine(d.i); } } public class yyy { public int i; public yyy( int j) { i = j; } public static yyy operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); yyy z = new yyy(x.i+y.i);
- 198. return z; } } Compiler Error a.cs(7,9):error CS0019: Operator ‘+’ cannot be applied to operands of type ‘yyy’ and ‘int’ C# is now telling you that you can add two yyy’s but cannot do the same, i.e. add a yyy and an int as we have not told C# how to do so. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); yyy d = a + b + 20; System.Console.WriteLine(d.i); } } public class yyy { public int i; public yyy( int j) { i = j; } public static yyy operator + ( yyy x , yyy y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y.i); yyy z = new yyy(x.i+y.i); return z; } public static yyy operator + ( yyy x , int y) { System.Console.WriteLine(“operator + “ + x.i + “ “ + y); yyy z = new yyy(x.i+y); return z; } } Output Operator + 10 5 Operator + 15 20 35 Life is fun in the fast lane. As we could have a large number of functions with the same name but differing number of parameters, ditto for operators. a + b as usual calls the first operator +. This as usual creates a temp object, say zz and now C# reads the line as zz + 20. This matches the second operator + which now gets called. You can have over a million plus operators with differing parameters if you desire. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); if ( a > b ) System.Console.WriteLine(“true”); else
- 199. System.Console.WriteLine(“false”); } } public class yyy { publicint i; public yyy( int j) { i = j; } public static bool operator > ( yyy x , yyy y) { System.Console.WriteLine(“operator < “ + x.i + “ “ + y.i); return x.i > y.i; } } Compiler Error a.cs(20,20): error CS0216: The operator ‘yyy.operator >(yyy, yyy)’ requires a matching operator ‘<‘ to also be defined C# is a romantic at heart and loves pairs. We tried to overload the > operator and C# tells us that we have to also overload the < operator. It makes sense as a user would want to know whether a yyy is greater than or less than another yyy. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); yyy b = new yyy(5); if ( a > b ) System.Console.WriteLine(“true”); else System.Console.WriteLine(“false”); } } public class yyy { public int i; public yyy( int j) { i = j; } public static bool operator > ( yyy x , yyy y) { System.Console.WriteLine(“operator > “ + x.i + “ “ + y.i); return x.i > y.i; } public static bool operator < ( yyy x , yyy y) { System.Console.WriteLine(“operator < “ + x.i + “ “ + y.i); return x.i < y.i; } } Output operator > 10 5 true
- 200. The operator <returns a bool as we would like to use it as part of an if or a while. In this case we are using it as part of an if and nothing stops us from using the < overloaded for ints and yyy’s to return a bool. What code you write in an operator is entirely your decision. The < operator looks like the + and can take different parameters. To overload the !=, you also have to overload the ==. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); System.Console.WriteLine(a); System.Console.WriteLine(a.ToString()); } } public class yyy { public int i; public yyy( int j) { i = j; } } Output yyy yyy In the first WriteLine function to be called, we are passing the object a whereas the first parameter to WriteLine should be a string. Also we do not get an error and we see yyy displayed. A longtime back we told you that all classes finally derive from object. The class object has a function ToString. Thus calling the ToString function off a produces the same output as the above line. We have not created the ToString function. So either we got a free ToString implementation from C# like we get a free Constructor or the ToString function of object in some way determines the name of our class and returns it as a string. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); System.Console.WriteLine(a); System.Console.WriteLine(a.ToString()); } } public class yyy { public int i; public yyy( int j) { i = j; } public string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Compiler Warning
- 201. a.cs(17,15): warning CS0114:‘yyy.ToString()’ hides inherited member ‘object.ToString()’. To make the current method override that implementation, add the override keyword. Otherwise add the new keyword. It is the same warning if a function exists in the base class. Output yyy ToString mukhi WriteLine displays yyy and a.ToString displays mukhi. Which means that they call different ToString functions. The WriteLine(a) calls the ToString of object whereas the second WriteLine calls it of yyy. If we want to override a function in the base class, we have to specifically do so by using the modifier override. If we do not, the default is to call the base class ToString. All this has been explained earlier, in any case. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); System.Console.WriteLine(a); System.Console.WriteLine(a.ToString()); } } public class yyy { public int i; public yyy( int j) { i = j; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Output ToString mukhi ToString mukhi Now both the System.Console.WriteLine calls the ToString function of yyy. We would like to convert a yyy into a string or an int for example. These type conversions are a major part of operator overloading. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); System.Console.WriteLine(a); } } public class yyy { public int i; public yyy( int j)
- 202. { i = j; } publicstatic implicit operator string(yyy y) { System.Console.WriteLine(“operator string”); return “string “ + y.i; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Output operator string string 10 In spite of having a function ToString, it does not get called unlike earlier. After the keyword operator we have the name of a data type i.e. string. This function is called whenever we want to convert our object into a string. The parameter y stands for the object to be converted. In this function we can write whatever code we want to but we must return a string. In our case we return the text string concatenated with the current value of i. Once again, the WriteLine function requires a string class. We are offering a yyy class. C# checks whether there is an operator string which is available to convert a yyy into a string. As there is one, it gets called. The word implicit means that we are indirectly implying that it should be called. Lets explain this with another example. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); string s; s = a; System.Console.WriteLine(s); } } public class yyy { public int i; public yyy( int j) { i = j; } public static explicit operator string(yyy y) { System.Console.WriteLine(“operator string”); return “string “ + y.i; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Compiler Error a.cs(7,5): error CS0029: Cannot implicitly convert type ‘yyy’ to ‘string’
- 203. We have changedthe word implicit which means imply. It also means giving a hint with the word explicit which means specify. Had we not changed implicit with explicit, we would have got no error and C# would have called the operator to convert a into a string and initialize s to it. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); string s; s = (string)a; System.Console.WriteLine(s); } } public class yyy { public int i; public yyy( int j) { i = j; } public static explicit operator string(yyy y) { System.Console.WriteLine(“operator string”); return “string “ + y.i; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Output operator string string 10 The () is called a cast and it takes any data type within brackets. We are explicitly asking for a conversion to a string and unlike earlier we are not being implicit but explicit. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); string s; s = (string)a; System.Console.WriteLine(s); System.Console.WriteLine(a); } } public class yyy { public int i; public yyy( int j) { i = j; } public static explicit operator string(yyy y) {
- 204. System.Console.WriteLine(“operator string”); return “string“ + y.i; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Output operator string string 10 ToString mukhi The explicit will get called only when we cast and as we do not have an implicit modifier, the ToString gets called. We get no error if we do not have an implicit modifier. a.cs public class zzz { public static void Main() { yyy a = new yyy(10); string s; s = (string)a; System.Console.WriteLine(s); System.Console.WriteLine(a); } } public class yyy { public int i; public yyy( int j) { i = j; } public static explicit operator string(yyy y) { System.Console.WriteLine(“operator string”); return “string “ + y.i; } public static implicit operator string(yyy y) { System.Console.WriteLine(“operator string”); return “string “ + y.i; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Compiler Error a.cs(24,15): error CS0557: Duplicate user-defined conversion in class ‘yyy’ We cannot have both the implicit and explicit modifier as two separate functions. Thus you have to decide which one you would want to implement. a.cs
- 205. public class zzz { publicstatic void Main() { yyy a ; a = 10; } } public class yyy { public int i; public yyy( int j) { i = j; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Output a.cs(7,5): error CS0029: Cannot implicitly convert type ‘int’ to ‘yyy’ One more error. Here we are trying to equate a yyy to an int but unlike earlier, we have not created an object like yyy by saying new. We would need someone to create the yyy object and initialize it to 10, an int. a.cs public class zzz { public static void Main() { yyy a ; a = 10; System.Console.WriteLine(a.i); } } public class yyy { public int i; public yyy( int j) { i = j; } static public implicit operator yyy(int v) { System.Console.WriteLine(“operator yyy int “ + v); yyy z = new yyy(v); return z; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Output operator yyy int 10 10
- 206. A constructor hasthe same name as the name of the class. Thus operator yyy may double up as a constructor. When we write a = 10, C# calls this constructor and passes the parameter v a value, 10. Operator yyy has now to create an object that looks like yyy and return it. Thus it does not act like a constructor in the sense that a constructor is responsible for creating the object. In other words, writing a = 10, creates a new object that looks like yyy and initializes it to 10. It is different from what we did earlier as previously the operator did not have to create an object that looks like yyy. In this case it has to. a.cs public class zzz { public static void Main() { yyy a ; a = “Hi”; System.Console.WriteLine(a.i); } } public class yyy { public int i; public yyy( int j) { i = j; } static public implicit operator yyy(int v) { System.Console.WriteLine(“operator yyy int “ + v); yyy z = new yyy(v); return z; } static public implicit operator yyy(string v) { System.Console.WriteLine(“operator yyy string “ + v); yyy z = new yyy(100); return z; } public override string ToString() { System.Console.WriteLine(“ToString”); return “mukhi”; } } Output operator yyy string Hi 100 We can have as many operator yyy functions as we like, provided we follow the rules of function overloading. Thus we have two of them, one that takes a string, another that takes an int. In this case as we are writing a = “hi”, the second operator that accepts a string get called. They all have to return an object that looks like yyy. a = (short) 10; Had we written the above line, we will not get any errors. We assumed we will get an error as we do not have an operator yyy which takes a short as a parameter. C# is highly intelligent. It first checks whether we have an operator yyy that matches what we wrote. In no case, there is a match. It will then check whether there is any other way it can prevent an error from being signaled. C# realizes that there is an operator yyy which accepts an int. Thus it converts a short to an int and then calls the operator yyy with the int parameter.
- 207. 13 Collection Objects Earlier weused an array in a foreach. If the array had three members, the foreach got executed three times and each time the variable i had a different value. The concept described below is called a collection class. It is a class that returns a value each time till the values run out, thus making it easier for us to iterate through the array. a.cs public class zzz { public static void Main() { yyy f = new yyy(); foreach (string i in f) { } } } class yyy { } Compiler Error a.cs(6,1): error CS1579: foreach statement cannot operate on variables of type ‘yyy’ because ‘yyy’ does not contain a definition for ‘GetEnumerator’, or it is inaccessible To use yyy in a foreach as a collection class, foreach requires a function GetEnumerator. a.cs public class zzz { public static void Main() { yyy f = new yyy(); foreach (string i in f) { } } } class yyy { public int GetEnumerator() { } } Compiler Error a.cs(6,1): error CS1579: foreach statement cannot operate on variables of type 'yyy' because 'int' does not contain a definition for 'MoveNext', or it is inaccessible a.cs(13,12): error CS0161: 'yyy.GetEnumerator()': not all code paths return a value
- 208. Foreach obviously triesto execute the function called GetEnumerator. This function should not return an int but something else as the error suggests. a.cs using System.Collections; public class zzz { public static void Main() { yyy f = new yyy(); foreach (string i in f) { } } } class yyy { public IEnumerator GetEnumerator() { return new xxx(); } } class xxx : IEnumerator { } Compiler Error a.cs(19,7): error CS0535: ‘xxx’ does not implement interface member ‘System.Collections.IEnumerator.MoveNext()’ a.cs(19,7): error CS0535: ‘xxx’ does not implement interface member ‘System.Collections.IEnumerator.Reset()’ a.cs(19,7): error CS0535: ‘xxx’ does not implement interface member ‘System.Collections.IEnumerator.Current’ IEnumerator is an interface which has three functions MoveNext, Reset and Current and xxx has to implement all of them to remove the compiler errors. a.cs using System.Collections; public class zzz { public static void Main() { yyy f = new yyy(); foreach (string i in f) { System.Console.WriteLine(i); } } } class yyy { public IEnumerator GetEnumerator() { return new xxx(); } } class xxx : IEnumerator
- 209. { public bool MoveNext() { System.Console.WriteLine(“MoveNext”); returntrue; } public void Reset() { System.Console.WriteLine(“Reset”); } public object Current { get { System.Console.WriteLine(“Current”); return “hi”; } } } Run the program and you will notice that the output does not stop. It goes on forever. IEnumerator is an interface which belongs to the namespace System.Collections. foreach first calls the function GetEnumerator from yyy. It expects this function to return an object like IEnumerator. It then calls the function MoveNext from this returned object. If MoveNext returns true it knows that there is some data to be read and it calls the property Current to access this data. From Current the get accessor gets called which always returns “hi” in our case. Then MoveNext gets called and if it returns false, we quit out of the foreach statement. As MoveNext always returns true, we go into an indefinite loop. a.cs using System.Collections; public class zzz { public static void Main() { yyy f = new yyy(); foreach (string i in f) { System.Console.WriteLine(i); } } } class yyy { public IEnumerator GetEnumerator() { return new xxx(); } } class xxx : IEnumerator { public string [] a = new string[3] {“hi” , “bye” ,”no”}; public int i = -1; public bool MoveNext() { i++; System.Console.WriteLine(“MoveNext” + i); if ( i == 3) return false; else return true; } public void Reset()
- 210. { System.Console.WriteLine(“Reset”); } public object Current { get { System.Console.WriteLine(“Current“ + a[i]); return a[i]; } } } Output MoveNext0 Current hi hi MoveNext1 Current bye bye MoveNext2 Current no no MoveNext3 We have created an array a which has 3 members and initialized them respectively to hi, bye and no by giving the strings in {} immediately after the new. Each time MoveNext gets called the variable i is increased by 1. If the value of i is 3, we have no more strings to return and thus we return false, else we return true. The variable i keeps track of how many times the function MoveNext is being called. As MoveNext returns true, Current gets called which returns a string from the array using i as the offset. Thus we can iterate through the entire array depending upon the length. a.cs using System.Collections; public class zzz { public static void Main() { yyy f = new yyy(“This is Great”); foreach (string i in f) { System.Console.WriteLine(i); } } } class yyy { string t; public yyy(string t1) { t = t1; } public IEnumerator GetEnumerator() { return new xxx(t); } } class xxx : IEnumerator { public string [] a; public xxx(string t3) {
- 211. char [] b= new char[1]; b[0] = ‘ ‘; a = t3.Split(b); } public int i = -1; public bool MoveNext() { i++; System.Console.WriteLine(“MoveNext “ + i); if ( i == a.Length) return false; else return true; } public void Reset() { System.Console.WriteLine(“Reset”); } public object Current { get { System.Console.WriteLine(“Current “ + a[i]); return a[i]; } } } Output MoveNext 0 Current This This MoveNext 1 Current is is MoveNext 2 Current Great Great MoveNext 3 Pretty big program. At the time of creating a yyy object we are passing a string to the constructor. Thus the yyy constructor gets called first. The constructor stores this string in variable t. The foreach statement calls GetEnumerator which now creates a xxx object passing it the string through t. The constructor of xxx now gets called. Every string class has a member function called Split. Split will break up a string on certain characters which we call delimiters. In this case, we want our string to be broken up whenever we encounter a space. The Split function requires an array of chars which it can use as a delimiter. The reason it requires an array is because we may have more than one char that we would like to break the string on. Like earlier, the array a now contains the array of strings. The last change is the condition in the if statement. Earlier we used a constant number, now we use a member, Length, of an array which stores the length of the array or the number of members. Thus the class yyy can now be used as a collection class which enumerates the individual words in the string. The function Reset for some reason never ever gets called.
- 212. 14 Attributes, The ReflectionAPI And Conditionals Attributes a.cs class zzz { public static void Main() { } } [vijay] class yyy { } Compiler Error a.cs(7,2): error CS0246: The type or namespace name 'vijay' could not be found (are you missing a using directive or an assembly reference?) Anything in a square bracket is called an attribute. We tried to create an attribute called vijay, which C#, for some reason, does not seem to recognize. a.cs class zzz { public static void Main() { } } class vijay : System.Attribute { } [vijay] class yyy { } All that we have done is created a class vijay that has been derived from the class System.Attribute and the error simply disappears. Thus an attribute is simply a class that derives from System.Attribute. To understand attributes lets take an example with structures. a.cs class zzz { public static void Main() {
- 213. yyy a =new yyy(); a.i = 65536+512+3; System.Console.WriteLine(a.i + " " + a.j + " " + a.k); } } struct yyy { public int i; public short j; public byte k; } Output 66051 0 0 A simple revision once again. We have created a structure a, that looks like yyy and initialized only one member i. Hence we see the warnings. The other members j and k get a default value of zero. a.cs using System; class zzz { unsafe public static void Main() { Console.WriteLine(sizeof(byte) + " " + sizeof(short) + " " + sizeof(int) + " " + sizeof(long)); } } >csc a.cs Compiler Error a.cs(4,27): error CS0227: Unsafe code may only appear if compiling with /unsafe The error here says that you have to use the /unsafe option while compiling any unsafe code. >csc a.cs /unsafe Output 1 2 4 8 We shall explain the modifier unsafe in the next chapter. Sizeof tells us how much memory C# allocates for a data type. A byte is allocated one memory location, short 2, int 4 and a long 8. a.cs using System.Runtime.InteropServices; class zzz { public static void Main() { yyy a = new yyy(); a.i = 65536+512+3; System.Console.WriteLine(a.i + “ “ + a.j + “ “ + a.k); } } [StructLayout(LayoutKind.Explicit)] struct yyy { [FieldOffset(0)] public int i; [FieldOffset(0)] public short j; [FieldOffset(0)] public byte k; }
- 214. Output 66051 515 3 Weare using an attribute StructLayout that belongs to the namespace System.Runtime.InteropServices. In the earlier program, we had used an attribute called vijay. Thus, StructLayout is a class derived from Attribute. We are passing a parameter LayoutKind.Explicit to it. The output now differs dramatically. Every variable is stored in memory. FieldOffset indicates the starting position of the variable within the memory location. Offset of 0 will position i, j, and k, all three variables at the same memory address of a. Explicit requires FieldOffset to be mentioned as we are explicitly laying the order for the variables held in the strucuture.LayoutKind.Sequential and LayoutKind.Auto gives different memory locations to each of the variable. We will explain the reasons a little later in the coming chapter ‘Unsafe Code’. We have seen how important attributes are so lets delve deeper into them. a.cs class zzz { public static void Main() { } } class vijayAttribute : System.Attribute { } [vijay] class yyy { } [vijayAttribute] class yyy1 { } We are allowed a little leeway in the name of the attribute. By convention, the attribute class should end with the word Attribute and when we use the attribute, the name attribute is optional. a.cs class zzz { public static void Main() { } } class vijay : System.Attribute { } [vijay("hi")] class yyy { } Compiler Error a.cs(10,2): error CS1501: No overload for method 'vijay' takes '1' arguments We had used the attribute StructLayout earlier where we passed a parameter. When we do the same thing with our attribute vijay, we get the above error. a.cs class zzz
- 215. { public static voidMain() { } } class vijay : System.Attribute { public vijay(string s) { } } [vijay("hi")] class yyy { } We forgot to add a constructor that accepts a string as a parameter. If we had passed a number to our attribute vijay, we would have to create a constructor that accepts an int. Thus if we pass 2 parameters to vijay, we need the appropriate constructor. a.cs class zzz { public static void Main() { } } class vijay : System.Attribute { public vijay(string s ,int i) { } } [vijay("hi",10,mukhi = 200)] class yyy { } Compiler Error a.cs(13,16): error CS0103: The name 'mukhi' does not exist in the class or namespace 'vijay' What we tried to do is, take a word called mukhi and initialize it to 200. C# comes back and tells us that it does not know what mukhi is. a.cs class zzz { public static void Main() { } } class vijay : System.Attribute { public vijay(string s ,int i) { } public int mukhi; } [vijay("hi",10,mukhi = 200)] class yyy { }
- 216. mukhi, now, iscalled a named parameter. It can also be termed as a property. a.cs class zzz { public static void Main() { } } class vijay : System.Attribute { public vijay(string s ,int i) { } public int mukhi; public string sonal { get { return "ss"; } set { ; } } } [vijay("hi",10, mukhi = 200, sonal = "bye")] class yyy { } A named parameter is a non-static field or a non-readonly property. A positional parameter is what we pass on to a constructor. We have 2 positional parameters as our constructor has two parameters and mukhi and sonal are our named parameters. The named parameters come after the positional ones. The positional parameter's order is important, but the named parameters can be in any order. If we don't follow this rule we will get an error as follows. Compiler Error a.cs(19,22): error CS1016: Named attribute argument expected When we place the attribute before a function, the error disappears. a.cs using System.Runtime.InteropServices; using System; class zzz { public static void Main() { } } [AttributeUsage(AttributeTargets.Class)] class vijay : System.Attribute { public vijay(string s ,int i) { } public int mukhi; public string sonal
- 217. { get { return"ss"; } set { ; } } } class yyy { [vijay("hi",10, sonal = "bye",mukhi = 200 )] public void abc() {} } Compiler Error a.cs(24,2): error CS0592: Attribute 'vijay' is not valid on this declaration type. It is valid on 'class' declarations only. AttributeUsage is one more attribute class derived from Attribute. It gives us the option to decide where the user can use the Attribute. The parameter in this case is class and hence we can use it only in front of a class and not in front of a method. The default is anywhere. The Reflection API Reflection or Introspection is when you look within to find out about your true self. In the same way we need a method by means of which, our program can find out all about a class. We need to know how many methods, properties etc while our program is executing or running. This distinction is important and we could always read the documentation if we wanted to know more about the functionality of a class. But, C# gives us a large number of functions that tell us the innards of a class. These functions put together have to be used in a certain way. The functions have to be called in a certain order and the parameters to them have to conform to certain data types. This concept is called an API or a Application Program Interface. In short, an API is how a programmer uses functions to get a desired result. a.cs using System; class zzz { public static void Main() { Type m; m = typeof(int); System.Console.WriteLine(m.Name + " " + m.FullName); m = typeof(System.Int32); System.Console.WriteLine(m.Name + " " + m.FullName); m = typeof(yyy); System.Console.WriteLine(m.Name + " " + m.FullName); } } class yyy { } Output Int32 System.Int32 Int32 System.Int32 yyy yyy Typeof is a keyword. It needs a class name as a parameter. In the first case, we are specifying a class called int. typeof returns an object that looks like Type. This class has two members Name, which gives the name of the class and FullName which is the name preceded with the name of the Namespace. When we use int as the name of the class, the member Name does not display int but Int32; we mentioned earlier int is an alias for a structure Int32 in the System namespace. This is what FullName tells us. We thus have visible proof that int is an alias for a structure. yyy is class belonging to no namespace and hence the Name and FullName members are similar.
- 218. a.cs using System; using System.Reflection; classzzz { public static void Main() { Type m = typeof(yyy); MemberInfo [] n; n = m.GetMembers(); Console.WriteLine(n.Length); foreach ( MemberInfo a in n) { Console.WriteLine(a.Name); } } } class yyy { public void abc() {} private int pqr( int i ) { return 0;} protected string xyz (string g , int p) {return "";} } Output 6 GetHashCode Equals ToString abc GetType .ctor We are now displaying the members of a class yyy. The class Type has a function called GetMembers that returns an array of type MemberInfo. Every array has a field called Length that returns the size of the array. In our specific case it is 6. We then use a foreach to run through each member of the MemberInfo array and are displaying the name of each function using the field Name from the class MemberInfo. a.cs using System; using System.Reflection; class zzz { public static void Main() { Type m = typeof(yyy); MemberInfo [] n; n = m.GetMembers(); Console.WriteLine(n.Length); foreach ( MemberInfo a in n) { Console.WriteLine((MemberInfo)a + " " + a.DeclaringType); } } } class yyy { public int i; public void abc() {} public int pqr( int i ) { return 0;} public string xyz (string g , int p) {return "";} } Output 9
- 219. Int32 i yyy Int32GetHashCode() System.Object Boolean Equals(System.Object) System.Object System.String ToString() System.Object Void abc() yyy Int32 pqr(Int32) yyy System.String xyz(System.String, Int32) yyy System.Type GetType() System.Object Void .ctor() yyy The first concept you need to be clear with is that we can inspect only details of public members and protected or private like pqr and xyz. Also variables are part of the members of a class. The MemberInfo object has a ToString function that displays the entire function in all its glory including parameters and their data types. The names of the parameter variables are however not being displayed. The DeclaringType member returns the class name that the member belongs to. Thus we can differentiate which class created the function. Let us now display the attributes used on a class. a.cs using System.Runtime.InteropServices; using System; using System.Reflection; class zzz { public static void Main() { Type m; m = typeof(yyy); System.Console.WriteLine(m.Name); foreach(object a in m.GetCustomAttributes (true)) Console.WriteLine(a); } } [AttributeUsage(AttributeTargets.All)] class vijay : System.Attribute { string s1,s2;int i1; public int mukhi; public override string ToString() { return s1+" " + s2+" " + i1 + " " + mukhi; } public vijay(string s ,int i) { s1=s;i1=i; } public string sonal { get { return s2; } set { s2 = value; } } } [vijay("hi1",10, sonal = "bye1",mukhi = 200 )] class yyy { [vijay("hi2",100, sonal = "bye2",mukhi = 2000 )] public void abc() {} [vijay("hi3",1000, sonal = "bye3",mukhi = 2 )] public int i; } Output
- 220. yyy hi1 bye1 10200 GetCustomAttributes takes a boolean as parameter and returns an array of objects. The ToString function of the attribute class gets called which will decide what string the attribute stands for. a.cs using System.Runtime.InteropServices; using System; using System.Reflection; class zzz { public static void Main() { Type m; m = typeof(yyy); System.Console.WriteLine(m.Name); foreach(MethodInfo a in m.GetMethods()) { object [] b = a.GetCustomAttributes(true); foreach(Attribute c in b) { Console.WriteLine(c); } } } } [AttributeUsage(AttributeTargets.All)] class vijay : System.Attribute { string s1,s2;int i1; public int mukhi; public override string ToString() { return s1+" " + s2+" " + i1 + " " + mukhi; } public vijay(string s ,int i) { s1=s;i1=i; } public string sonal { get { return s2; } set { s2 = value; } } } [vijay("hi1",10, sonal = "bye1",mukhi = 200 )] class yyy { [vijay("hi2",100, sonal = "bye2",mukhi = 2000 )] public void abc() {} [vijay("hi3",1000, sonal = "bye3",mukhi = 2 )] public void pqr() {} } Output yyy hi2 bye2 100 2000 hi3 bye3 1000 2 The object m looks like Type. As explained earlier, we are calling a function called GetMethods which returns an array of MethodInfo's. a loops through each one. We have two methods and the foreach gets executed twice.
- 221. Once for abcand then for pqr. The GetCustomAttributes also exists in a MethodInfo class that returns an array of objects representing our attributes. We iterate through each, displaying what the ToString function returns. As we have only one attribute per function, the second for each gets executed only once. a.cs using System.Runtime.InteropServices; using System; using System.Reflection; class zzz { public static void Main() { Type m; m = typeof(yyy); System.Console.WriteLine(m.Name); foreach(MethodInfo a in m.GetMethods()) { object [] b = a.GetCustomAttributes(true); foreach(Attribute c in b) { if ( c is vijay ) Console.WriteLine(c); } } } } [AttributeUsage(AttributeTargets.All)] class vijay : System.Attribute { string s1,s2;int i1; public int mukhi; public override string ToString() { return s1+" " + s2+" " + i1 + " " + mukhi; } public vijay(string s ,int i) { s1=s;i1=i; } public string sonal { get { return s2; } set { s2 = value; } } } [vijay("hi1",10, sonal = "bye1",mukhi = 200 )] class yyy { [vijay("hi2",100, sonal = "bye2",mukhi = 2000 )] public void abc() {} [vijay("hi3",1000, sonal = "bye3",mukhi = 2 )] public void pqr() {} } There is no change at all in the output. A function can be decorated with as many attributes as you like. We would like to filter out certain attributes. a.cs using System.Runtime.InteropServices; using System; using System.Reflection; class zzz
- 222. { public static voidMain() { Type m; m = typeof(yyy); System.Console.WriteLine(m.Name); foreach(MethodInfo a in m.GetMethods()) { object [] b = a.GetCustomAttributes(true); foreach(Attribute c in b) { if ( c is vijay ) Console.WriteLine(c); } } } } class vijay : System.Attribute { public override string ToString() { return "vijay"; } } class vijay1 : System.Attribute { public override string ToString() { return "vijay"; } } class yyy { [vijay()] public void abc() {} [vijay()] [vijay1()] public void pqr() {} } Output yyy vijay vijay We have two attribute classes vijay and vijay1. The function pqr has been decorated with 2 attributes whereas abc with only one. However we do not see vijay1 in the output as the 'c is vijay' makes the if statement true only for the attribute vijay and not vijay1. For the function pqr GetCustomAttributes returns an array of size two, but the if statement is true only for one of them, the one with the attribute name vijay. This is because of the 'is'. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); if ( a is yyy) Console.WriteLine("a yyy"); xxx b = new yyy(); if ( b is xxx) Console.WriteLine("b xxx");
- 223. if ( bis yyy) Console.WriteLine("b yyy"); int d = 10; if ( d is yyy) Console.WriteLine("b yyy"); } } class xxx { } class yyy : xxx { } Output a yyy b xxx b yyy We would like to know the data type of an object at runtime. C# offers you a keyword 'is' that lets you check the data type of an object. a looks like yyy and 'is' results in true. B looks like xxx but is initialized to a new yyy. Thus it doubles up for a yyy and a xxx resulting in the next two is's returning true. D is an int and not a yyy, so the last 'is' is false. Attributes Revisited Positional parameters are a must whereas names parameters are optional. Attribute parameters can be a bool, byte, char, short, int, long, float and double. These are the simple types that C# supports. Other data types are string, enums, objects arrays etc. Attribute usage has a position parameter which specifies the elements where the attribute can be used. The default is All. It also has one named parameter called AllowMultiple. a.cs using System.Runtime.InteropServices; using System; class zzz { public static void Main() { } } [AttributeUsage(AttributeTargets.All)] class vijay : System.Attribute { public vijay(string s) { } } class yyy { [vijay("hi")][vijay("hi1")] public void abc() {} } Compiler Error a.cs(18,15): error CS0579: Duplicate 'vijay' attribute By default we cannot use the same attribute twice on any entity.
- 224. a.cs using System.Runtime.InteropServices; using System; classzzz { public static void Main() { } } [AttributeUsage(AttributeTargets.All,AllowMultiple=true)] class vijay : System.Attribute { public vijay(string s) { } } class yyy { [vijay("hi")][vijay("hi1")] [vijay("hi2") , vijay("hi3")] public void abc() {} } We get no error as by default the AllowMultiple named parameter has a value of false. If we set its value to true, we are allowed to use multiple attributes on any entity. The above two forms are similar and either one can be used. Attribute permits us to set declarative information for various program entities for use by someone else at run time. Conditionals a.cs using System.Diagnostics; using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { [Conditional("vijay")] public void abc() { Console.WriteLine("abc"); } } When we run the above program we get no output at all. In other words the function abc does not get called at all. This is inspite of writing a.abc(). a.cs #define vijay using System.Diagnostics; using System; class zzz { public static void Main() {
- 225. yyy a =new yyy(); a.abc(); } } class yyy { [Conditional("vijay")] public void abc() { Console.WriteLine("abc"); } } Output abc Any line beginning with a # is read by the C# pre-processor, a program that starts before the C# compiler starts. It has words like #define which creates a variable or word called vijay. In a programming language, a variable has to have a value, but in the preprocessor scheme of things, it may/maynot have a value. However if we do not give it a value, like in this case, the variable is only set to have been defined or created. Anything in [] brackets is an Attribute class. Earlier we had not created a variable vijay and hence the entire code of abc was left out of the executable file. Not only that, but all calls to function abc were eliminated from our code. All of this by defining or not defining a variable vijay. This is what we passed as the attribute to Conditional. We can create functions that are omitted during compilation depending upon a preprocessing symbol. When we write code, we add a lot of code for debugging purposes. This code is to help the programmer debug code. After a function works, it is error free, we do not require any of this debugging code. One way to eliminate this debugging code is by making the functions conditional. The resulting code is called a 'Retail build' and the debugging version, obviously a 'Debug build'. A Retail build is much smaller in size and obviously much faster. The #define has to be at the start of code. Another way of achieving the same result is by eliminating the #define from the code and creating a preprocessor symbol as an option to the compiler. >csc a.cs /d:vijay The compiler option is /d and the colon is part of the syntax. Following the colon is the name of the preprocessor symbol. In this case, it is vijay. We’ve discussed preprocessors in one of the earlier chapters. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { [Obsolete] public void abc() { Console.WriteLine("abc"); } } Compiler Error a.cs(7,1): warning CS0612: 'yyy.abc()' is obsolete Output
- 226. abc Many a timeswe create functions in a class which we would not want the user to use, as these functions were useful years ago, but are now obsolete. The only way to warn the user that some time in the future we will no longer support these functions is by marking them with the attribute Obsolete. We see the warning as displayed above but the program runs as normal. 15 Unsafe code Today, the programming language C is the most widely used because of only one reason and that is the use of pointers. In this chapter, we will explain what pointers are all about and how they can be used in the world of C#. a.cs class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { public void abc() { int *i; } } Compiler Error a.cs(13,6): error CS0214: Pointers may only be used in an unsafe context All simple variables like int, byte, short store numbers. When we create a variable in C#, we are allowed to put a multiplication/asterisk sign '*' in front of the variable. These variables are called Pointers. We get an error as C# considers pointers to be unsafe and hence we need special permission to use pointers. a.cs class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() {
- 227. int *i; } } Compiler Error a.cs(11,20):error CS0227: Unsafe code may only appear if compiling with /unsafe As seen in the previous chapter, the unsafe option must be tagged while compiling the program. Give the command as csc a.cs /unsafe and the error now disappears. a.cs class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } unsafe class yyy { public void abc() { int *i; } } The C# documentation very clearly states that the modifier can be used along with the class keyword, By using the modifier unsafe, we are asking C# to let us use pointers as we are unable to write code without the use of pointers. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { Console.WriteLine(sizeof(byte *) + " " + sizeof(short *) + " " + sizeof(int *) + " " + sizeof(long *)); Console.WriteLine(sizeof(byte **) + " " + sizeof(short **) + " " + sizeof(int **) + " " + sizeof(long **)); } } Output 4 4 4 4 4 4 4 4 Some time back, we had used the sizeof keyword, to determine how much memory C# allocates for our variables. When we ask for the sizeof an pointer variable, we always get 4, irrespective of whether the data type is a short or int or whatever, including another pointer. a.cs using System; class zzz {
- 228. public static voidMain() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { int *i; int j=1; i = &j; Console.WriteLine((int)i); *i = 10; Console.WriteLine(j); } } Output 1243472 10 We have created a variable j that has been initialized to one. We have also created a variable i, with a multiplication sign at the time of creation. By doing this C# allocated four memory locations to store i. What is most important is that a pointer variable is just like any another variable. It simply stores numbers. The single big difference is that a pointer variable value is interpreted as a computer memory location. If both i and j were being initialized to 1, then j is the number 1 as we know it, whereas i stand for computer memory location 1. Pointers can only be initialized to a computer memory location. Whenever we place an ampersand(&) in front of a variable, we are asking C# to tell us about the memory allocated for the variable. Every time C# creates a variable, it stores it somewhere in memory. We are at times interested in knowing where in memory the variable was allocated. A & in front of j will tell us the memory location where j starts in memory. As it is a computer memory location or address we can store it in i. C# allocated 4 memory location for j and the start of these 4 are being stored in a pointer. We would like to display the value of the pointer. The WriteLine function does not have an overload to display a pointer and we have to cast it to an int. As we get 1243472 displayed, it could only mean that the variable j begins here and is spread over memory locations 1243473, 1243474 and 1243475.
- 229. We can onlyplace the multiplication sign in front of a variable defined to be a pointer. If i is not a pointer, *i will return an error. C# here asks a simple question. What is the value of the variable i? . The answer is 1243472. C# will now go to memory location 1243472 to 1243475 and place the value 10 there. As the value of j is determined by what is present from memory locations 1243472 to 1243475, the value of j changes from 1 to 10. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { int *i; int j=1, k = 1; i = &j; Console.WriteLine((int)i); *i = 10; Console.WriteLine(j); i = &k; Console.WriteLine((int)i); *i = 100; Console.WriteLine(k + " " + j); } } Output 1243468 10 1243464 100 10
- 230. Whenever we createa pointer variable, int *i, we are not stating explicitly which int i will be pointing to. Thus it can point to one int today, another int tomorrow. This is what gives pointers their flexibility. It can point to any int it wants to in memory. We are first initializing i to point to j, then we are changing the value of j to 10 through the pointer. Then i points to k in memory, and then k's value is indirectly being changed to 100 through the pointer. The first WriteLine tells us that the variable j begins at memory location 1243468. From here the next 4 have been reserved for the variable j. Memory for k has been allocated at 1243464. Thus, physically, k starts first in memory from 1243464 to 67 and j from 1243468 onwards. a.cs using System;
- 231. class zzz { public staticvoid Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { int *i; int j=1; i = &j; Console.WriteLine((int)i + " " + (int) &i); } } Output 1243464 1243468 Lets not forget that pointers are also variables and C# allocates memory for i. An & in front of any variable tells us where it starts in memory. Thus i starts from memory location 1243468 onwards. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { long *i; long j=1, k = 1; i = &j; Console.WriteLine((int)i); i = &k; Console.WriteLine((int)i); } } Output 1243460 1243468 The sizeof a long is 8 bytes and the addresses printed differ by 8 bytes. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc();
- 232. } } class yyy { unsafepublic void abc(){ byte *i; byte j=1, k = 1; i = &j; Console.WriteLine((int)i); i = &k; Console.WriteLine((int)i); } } Output 1243468 1243472 The only change here is that j and k are not longs but bytes. Why are we learning all about pointers is a question you should ask yourself. The C# documentation says very clearly that a byte occupies one memory location. The sizeof byte will also return one. In the above output, however, the memory locations differ by 4. The reason being that on the Pentium Processor, if we want one memory location, we will receive a minimum of 4. That's why, the Pentium is said to be a 32-bit processor. It will give you memory in chunks of 4. From this knowledge, using a byte instead of a int does not conserve memory or speed up your program. The above explanation cannot be comprehended without the knowledge of pointers. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { byte *i; i = 0; Console.WriteLine((int)i); } } Compiler Error a.cs(15,5): error CS0029: Cannot implicitly convert type 'int' to 'byte*' C# is a strongly typed language and we are not allowed to convert a zero which is an int to a byte *, the data type of i. The only way out is to use a cast. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc()
- 233. { byte *i;char *j;int*k;long *l; i = (byte *)0; j = (char *)0; k = (int *)0 ; l = (long *)0; Console.WriteLine((int)i + " " + (int) j + " " + (int) k + " " + (int)l); i++;j++;k++;l++; Console.WriteLine((int)i + " " + (int) j + " " + (int) k + " " + (int)l); i++;j++;k++;l++; Console.WriteLine((int)i + " " + (int) j + " " + (int) k + " " + (int)l); } } Output 0 0 0 0 1 2 4 8 2 4 8 16 A pointer to any data type is allocated four memory locations at the time of creation. This enables pointers to store values that range from 0 to 4 billion. The question that comes to your mind, is what is the difference between a pointer to an int from a pointer to a long. We have created four variables i, j, k, l. Each is a pointer to a different data type. We have also initialized each variable to 0 and displayed their values using the WriteLine function. We have then incremented each of them by 1. To our surprise the pointer to an int k, increases by 4 and not by 1. The char pointer increases by 2 and long by 8. To make sure that is not a isolated phenomena, we increment them again. Same answer once again. They do not increase by one but by the size of the data type they point to. The sizeof a long is 8 and thus a pointer to a long increases by 8 and not 1. When we initialized l to zero, we were telling C# that a long begins at memory location 0. This long will occupy the next 8 memory locations, form 0 to 7. Thus when we write l++, we are telling C# to take us to the next long in memory, which has to begins now at 8. The first difference between pointers to different data types is that the amount the pointer value increases is dependent upon what it is pointing to. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void pqr(int x , int y , int z) { int *p1; int *p2; int *p3; p1 = &x; p2 = &y ; p3 = &z; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); *p1 = 20; *p2 = 30; } unsafe public void abc() { int *i ; int *j ; int *k; int l=1,m=2,n=3; i=&l; j=&m; k=&n; Console.WriteLine((int) i + " " + (int) j + " " + (int) k); pqr(l,m,n); Console.WriteLine(l + " " + m + " " + n); }} Output 1243460 1243456 1243452 1243424 1243436 1243432 1 2 3
- 234. We have createda variable i of type int in memory. It begins at memory location 1243460. Similarly m and n start at 56 and 52 respectively. We are then calling a function pqr and passing three parameters to it. These are being stored at memory locations 1243424, 36 and 32 respectively. When we write *p1, we are going to memory location 1243424 and writing 20 there. This will change the value of x from 1 to 20. As variable l begins at 1243460, its value remains unchanged. The area of memory where x, y and z start is called the stack. Stack memory is used to pass parameters to functions. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void pqr(int x , int y , int z) { int *p1; int *p2; int *p3; p1 = &x; p2 = &y ; p3 = &z; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); } unsafe public void xyz(int x , int y , int z) { int *p1; int *p2; int *p3; p1 = &x; p2 = &y ; p3 = &z; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); } unsafe public void abc() { int *i ; int *j ; int *k; int l=1,m=2,n=3; i=&l; j=&m; k=&n; Console.WriteLine((int) i + " " + (int) j + " " + (int) k); pqr(l,m,n); xyz(l,m,n); } } Output 1243460 1243456 1243452 1243424 1243436 1243432 1243424 1243436 1243432 Now you will begin to comprehend the importance of pointers. Its is an understanding of pointers that will give you a better insight into understanding the innards of a programming language. The last two WriteLines display the same answer as the stack memory gets reused for every function call. The stack is an area of memory that will store parameters and variables created in a function. Let us assume that the stack begins at memory location 100. The parameters are first pushed onto the stack and then all the local variables are created later, but below the parameters. Remember that the stack grows down in memory. When the function is over, the stack is moved back to 100, and the next function that gets called, reuses the same memory from 100. Thus anything created in a function has a lifetime of the open and close braces as the next function uses the same memory and the earlier values get overwritten. a.cs using System; class zzz
- 235. { public static voidMain() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void xyz() { int i=0,j=0,k=0; int *p1; int *p2; int *p3; p1 = &i; p2 = &j ; p3 = &k; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); } unsafe public void aaa() { int x=0,y=0,z=0; int *p1; int *p2; int *p3; p1 = &x; p2 = &y ; p3 = &z; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); } unsafe public void pqr() { int i=0,j=0,k=0; int *p1; int *p2; int *p3; p1 = &i; p2 = &j ; p3 = &k; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); } unsafe public void abc() { pqr(); xyz(); aaa(); } } Output 1243460 1243464 1243468 1243460 1243464 1243468 1243460 1243464 1243468 The above program proves a number of points. C# reuses the same memory for passing parameters and creating variables in functions. The variable names are not important as they get created on the stack, at the same place in memory. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void pqr(int x , int y , int z) {
- 236. int *p1; int*p2; int *p3; p1 = &x; p2 = &y ; p3 = &z; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); p3++;p3++;p3++;p3++;p3++;p3++; Console.WriteLine((int) p3); *p3=20; p3++; Console.WriteLine((int) p3); *p3=200; } unsafe public void abc() { int *i ; int *j ; int *k; int l=1,m=2,n=3; i=&l; j=&m; k=&n; Console.WriteLine((int) i + " " + (int) j + " " + (int) k); pqr(l,m,n); Console.WriteLine(l + " " + m + " " + n); } } Output 1243460 1243456 1243452 1243424 1243436 1243432 1243452 1243456 1 200 20 Pointers are really unsafe. There is a simple rule in C# which says that variables created in one function abc, cannot be changed by another function pqr. The variables m and n begin at memory locations 1243456 1243452 respectively. If there was some way, I could write to these memory locations, I would be changing the values of m and n respectively. The pointer p3 stores, where the parameter z is stored in memory i.e. 1243432. If we can increment this pointer 6 times, 6 will actually be 24 due to pointer arithmetic, the value of p will now be 1243452 which is the address of n in memory. Thus from one function, I can change the value of another variable. In this case we are doing it on purpose but if I called a function, where a pointer went haywire, my variables change and nobody knows why the program stops working. Thus pointers are like a sharp knife, it can save a life in the hands of a doctor but in the wrong hands it can also kill. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void pqr(int x , int y , int z) { int *p1; int *p2; int *p3; p1 = &x; p2 = &y ; p3 = &z; Console.WriteLine((int) p1 + " " + (int) p2 + " " + (int) p3); int q = 1; while ( q <= 12 ) { p1++; Console.WriteLine((int) p1 + " " + *p1); q++; } }
- 237. unsafe public voidabc() { int *i ; int *j ; int *k; int l=1,m=2,n=3; i=&l; j=&m; k=&n; Console.WriteLine((int) i + " " + (int) j + " " + (int) k); pqr(l,m,n); Console.WriteLine(l + " " + m + " " + n); } } Output 1243460 1243456 1243452 1243424 1243436 1243432 1243428 47645012 1243432 3 1243436 2 1243440 0 1243444 1243508 1243448 1243544 1243452 3 1243456 2 1243460 1 1243464 1243452 1243468 1243460 1243472 1243484 1 2 3 Would it not be a great idea to be able to display whatever is there on the stack. Between variables there are some gaps and the above program is displaying the contents of the stack. The variable q makes the loop go on 12 times and each time we increment p1 by 4. P1 is pointing to x the first parameter on the stack. Would it not aid understanding if Microsoft explained what all it pushes on the stack. One of the most common hacking exploits on the net is called a stack overflow. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } public class yyy { unsafe public void pqr( int *a) { Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]); } unsafe public void abc() { int [] a = new int[3]; a[0] = 10; a[1] = 2; a[2] = 30; pqr(a); } } Compiler Error a.cs(20,1): error CS1502: The best overloaded method match for 'yyy.pqr(int*)' has some invalid arguments a.cs(20,5): error CS1503: Argument '1': cannot convert from 'int[]' to 'int*'
- 238. We were tryingto pass an array as a parameter to a function. Unfortunately C# has some other views. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } public class yyy { unsafe public void pqr( int *a) { Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]); } unsafe public void abc() { int [] a = new int[3]; a[0] = 10; a[1] = 2; a[2] = 30; fixed ( int *i = a) pqr(i); } } Output 10 2 30 The keyword fixed removed the error. An array in memory can be moved around by C#. We would like C# to keep it fixed in memory. The keyword fixed guarantees that C# will not move it around in RAM for the duration of the program. The only reason C# moves objects in memory is to speed up execution of the program. The keyword fixed as part of syntax wants the () .Within them you can create a variable in our case i which we initialize to a C# object, an array a. The scope or lifetime of this array is in the next statement i.e. the invocation of function pqr. In case you have more lines you can use the {}. Even though a now is a pointer to an int, the notation a[0] refers to the first member. The name of an array tells us where the array starts in memory. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } public class yyy { unsafe public void pqr( int *a) { Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]); a[0] = 100 ; a[1] = 40; *(a+2) = 33; } unsafe public void abc() { int [] a = new int[3]; a[0] = 10; a[1] = 2; a[2] = 30; fixed ( int *i = a) pqr(i); Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]); } }
- 239. Output 10 2 30 10040 33 Changing anything through a pointer changes the original. Whenever we write a[1]= , we are changing the first member of the original array. a[1] is only a notation, it actually get converted to *(a+1). Thus we are changing the original members of the array. Also the array grows upward in memory. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } public class yyy { unsafe public void pqr( int *a) { Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]); for ( int i = 0 ; i<= 2000; i++) *(a+i) = i*10; } unsafe public void abc() { int [] a = new int[3]; a[0] = 10; a[1] = 2; a[2] = 30; fixed ( int *i = a) pqr(i); Console.WriteLine("{0} {1} {2}",a[0],a[1],a[2]); } } Please do not run the above program as the error would be unpredictable. The reason being that we are writing beyond the bounds of the array. The array a created in the function abc has only 12 memory locations allocated for it. We are writing the next 2000 * 4, 8000 memory locations. The error on your machine will be very different form another machine. This unpredictability is why pointer errors are hard to catch, and your boss would thus not like you to use a pointer while programming. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } struct aaa { public int i,j; } class yyy { unsafe public void abc() { aaa *a;aaa b; b = new aaa();
- 240. a = (aaa*)b; } } Compiler Error a.cs(20,10): error CS0030: Cannot convert type 'aaa' to 'aaa*' aaa is a structure with two members i and j. new aaa() allocates memory for i and j. new aaa() and b are of the same data type ie aaa. When we try to equate a and b, C# canot convert a pointer to a struct/class to the object itself. Remember new returns not a pointer to an object but the object itself. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } struct aaa { public int i,j; } class yyy { unsafe public void abc() { aaa *a;aaa b; b = new aaa(); a = &b; int *x; x = &(a->i); int *y; y = &(a->j); Console.WriteLine((int) a + " " + (int)x + " " + (int) y); } } Output 1243468 1243468 1243472 We need to take the address of where b begins in memory by using the & and equate that to a. The address of an object like aaa is of the data type aaa *. As a now is a pointer to a structure, we have to use a different syntax to display the members of the structure. The new way is to use the -> operator. Thus a->i refers to the member i of the structure that looks like aaa. The variable x now stores the address of where the first member i begins in memory. Obviously the address of the first member i and the address of the structure will be the same and the second member will be stored 4 later. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } struct aaa {
- 241. public int i,j; } classyyy { unsafe public void abc() { aaa *a;aaa b; b = new aaa(); a = &b; Console.WriteLine((int) a); b = new aaa(); a = &b; Console.WriteLine((int) a); } } Output 1243468 1243468 We have one object b that we have initialized twice with new. Each time we get the same memory location as the address of b in memory will remain the same. Unions a.cs using System.Runtime.InteropServices; class zzz { unsafe public static void Main() { yyy a = new yyy(); a.i = 65536+512+3; System.Console.WriteLine(a.i + " " + a.j + " " + a.k); int *i1;short *j1;byte *k1; i1 = &a.i; j1 = &a.j ; k1 = &a.k; System.Console.WriteLine((int)i1 + " " + (int) j1 + " " + (int) k1); System.Console.WriteLine(sizeof(yyy)); } } [StructLayout(LayoutKind.Explicit)] struct yyy { [FieldOffset(0)] public int i; [FieldOffset(0)] public short j; [FieldOffset(0)] public byte k; } Output 66051 515 3 1243476 1243476 1243476 4
- 242. Let us understandwhat a union is all about. We are printing the addresses of i, j and k and lo and behold they are all the same. In a union all the members begin at the same place and that is why their addresses were similar. If we understand pointers, understanding unions then becomes a piece of cake as we can visually see the addresses. a.cs using System.Runtime.InteropServices; class zzz { unsafe public static void Main() { yyy a = new yyy(); a.i = 65536+512+3; System.Console.WriteLine(a.i + " " + a.j + " " + a.k); int *i1;short *j1;byte *k1; i1 = &a.i; j1 = &a.j ; k1 = &a.k; System.Console.WriteLine((int)i1 + " " + (int) j1 + " " + (int) k1); System.Console.WriteLine(sizeof(yyy)); } } [StructLayout(LayoutKind.Explicit)] struct yyy { [FieldOffset(0)] public int i; [FieldOffset(0)] public short j; [FieldOffset(10)] public byte k; } Output 66051 515 0 1243468 1243468 1243478 11 a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { short i = 512+3;
- 243. byte *j; j =(byte *)&i; Console.WriteLine((int)j + " " + i ); *j = 1; Console.WriteLine(i); j++; Console.WriteLine((int)j); *j=1; Console.WriteLine(i); j++; Console.WriteLine((int)j); *j=10; Console.WriteLine(i); } } Output 1243468 515 513 1243469 257 1243470 257
- 244. j is apointer to a byte. From the output, we can see that i begins at 1243468. i is of data type short which means it takes 2 bytes. j now contains the the address value of i i.e. 1243468. The first line proves it. The statement *j=1 will change the bottom byte value to 1.j++ will then increment the address value by 1, hence *j=1 will now change the top byte to 1. The last j++ has no effect because the scope of short is only 2 btes. Hence we see no change in the value of i. Here, in place of the data type short, we have given an int. a.cs using System; class zzz { public static void Main() { yyy a = new yyy(); a.abc(); } } class yyy { unsafe public void abc() { int i = 512+3; byte *j; j = (byte *)&i; Console.WriteLine((int)j + " " + i ); *j = 1; Console.WriteLine(i); j++; Console.WriteLine((int)j); *j=1; Console.WriteLine(i); j++; Console.WriteLine((int)j); *j=10; Console.WriteLine(i);
- 245. } } Output 1243468 515 513 1243469 257 1243470 655617 The lastj++ in the series will touch the third byte, there by replacing the 1 to 10. Hence the output changes dramatically.