2 programming with c# i

Chapter Three
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C# fundamentals

Objectives
 After the end this lesson the student will be able to
 Understand C# language fundamentals
> Data type, variables and constants, …
 Write a C# program statement
 Debug C# program in VS
 Develop OO program with C#
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Lesson Outline
 C# fundamentals
 Data types, variables and constants
 Comments in C#
 Namespace
 Type casting
 Data type conversion
 Operators and Expressions
 Console Input / Output
 C# Code control structure
 Conditional statements
 Looping statements
 Jump statements
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C#(C-Sharp)
 Microsoft C# (pronounced See Sharp) developed by Microsoft
Corporation, USA
 New programming language that runs on the .NET Framework
 C# is simple, modern, type safe, and object oriented
 C# code is compiled as managed code
 Combines the best features of Visual Basic, C++ and Java
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C# Features
 Simple
 Modern
 Object-Oriented
 Type-safe
 Versionable
 Compatible
 Secure
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Data Types
 are sets (ranges) of values that have similar characteristics
 Data types are characterized by:
 Name – for example, int;
 Size (how much memory they use) – for example, 4 bytes;
 Default value – for example 0.
 two major sets of data types in C#
 value types
> store their own data
 reference types
> Store a reference to the area of memory where the data is stored
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Data Types …
 Basic data types in C# :
 Integer types – sbyte, byte, short, ushort, int, uint, long, ulong;
 Real floating-point types – float, double;
 Real type with decimal precision – decimal;
 Boolean type – bool;
 Character type – char;
 String – string;
 Object type – object.
 These data types are called primitive (built-in types),
 because they are embedded in C# language at the lowest level.
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Data Types …
 C# Keyword Bytes .Net Type default Min value Max value
 sbyte 1 SByte 0 -128 127
 byte 1 Byte 0 0 255
 short 2 Int16 0 -32768 32767
 ushort 2 UInt16 0 0 65535
 int 4 Int32 0 -2147483648 2147483647
 uint 4 UInt32 0u 0 4294967295
 long 8 Int64 0L -9223372036854775808 9223372036854775807
 ulong 8 UInt64 0u 0 18446744073709551615
 float 4 Single 0.0f ±1.5×10 -45 ±3.4×1038
 double 8 Double 0.0d ±5.0×10 -324 ±1.7×10308
 decimal 16 Decimal 0.0m ±1.0×10-28 ±7.9×1028
 bool 1 Boolean false Two possible values: true and false
 char 2 Char 'u0000' 'u0000' 'uffff‘
 object - Object null - a reference to a String object
 string - String null - a reference to any type of object
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C# > Variables
A named area of memory
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Variables
 a named area of memory
 stores a value from a particular data type, and
 is accessible in the program by its name.
 Stores a value that can change as the program executes
 Before use it must be declared
 Variable declaration contains
 Data types
 Name
 Initial value
 Valid C# variable name
 Start with A-Z or a-z or _
 Can include number and _
 cannot coincide with a keyword
> Use "@". For example, @char and @null
 C# is case sensitive
 E.g. salary, number1, total_mark
 Naming convention
 camelNotation
> E,g. letterGrade
 PascalNotation
> E.g. LetterGrade
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Variables > keywords
 Reserved words by the language for some purpose
 Can’t be used by programmer for other purpose except they are reserved for
abstract as base bool break byte
case catch char checked class const
continue decimal default delegate do double
else enum event explicitextern false
finally fixed float for foreach goto
if implicitin int interface internal
is lock long namespace new null
object operator out override params private
protected public readonly ref return sbyte
sealed short sizeof stackalloc static string
struct switch this throw true try
typeof uint ulong unchecked unsafe ushort
using virtual void volatilewhile …
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Variables > Declaration
 Variable declaration and initialization
Syntax
type variableName;
variableName=value;
 Several ways of initializing:
 By using the new keyword
 By using a literal expression
 By referring to an already initialized variable
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Variables > Example
 Example
int num = new int(); // num = 0
int x; //declaration statement
x=0; //assignment statement
char grade=‘A’; //enclose a character value in single quotes
double price = 10.55; //declare and initialize a value with 1 stmt
double scientificNumber= 3.45e+6; //scientific notation
decimal salary = 12345.67m; //m or M indicates a decimal value,
monetary(money)
float interestRate = 5.25f; //f or F indicates a float value
bool isValid = false; //declare and initialize a value with 1 stmt
double scgpa = 3.2, cgpa=3.12; //initialize 2 variables with 1 stmt
string greeting = "Hello World!"; //enclose a string value in double quotes
string message = greeting;
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Variables > Constant variables
 Constant
 Stores a value that can’t be changed as the program executes
 Always declared and initialized in a single statement
 Declaration statement begins with const keyword
 Capitalize the first letter in each word of a constant also common
practice – PascalNotation
 Example
const double Pension = 0.06;
const int DaysInWeek = 7;
const int Km = 100;
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Variables > String data type
 Strings are sequences of characters.
 declared by the keyword string
 enclosed in double quotation marks
 default value is null
 Example
string firstName = “Abebe”;
string lastName = “Kebede”;
string fullName = firstName + “ “ + lastName;
 Various text-processing operations can be performed using strings:
 concatenation (joining one string with another),
 splitting by a given separator,
 searching,
 replacement of characters and others.
 + concatenate, += append, More in working with Date and String class
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Variables > Object Type
 Object type is a special type
 which is the parent of all other types in the .NET Framework
 Declared with the keyword object,
 it can take values from any other type.
 It is a reference type,
> i.e. an index (address) of a memory area which stores the actual value.
 Example
object object1 = 1;
object object2 = “Two";
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Variables > Nullable Types
 A nullable Type is a data type is that contain the defined data type or
the value of null.
 are specific wrappers around the value types (as int, double and
bool) that allow storing data with a null value.
 provides opportunity for types that generally do not allow lack of
value (i.e. value null)
 to be used as reference types and to accept both normal values and the
special one null.
 Any DataType can be declared nullable type with the help of operator "?".
 Thus nullable types hold an optional value.
 Wrapping a given type as nullable can be done in two ways:
 nullabl <int> i1=null
 int? i2 = i1;
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Variables > Nullable Types – Example
int? someInteger = null;
Console.WriteLine("This is the integer with Null value -> " + someInteger);
someInteger = 5;
Console.WriteLine( "This is the integer with value 5 -> " + someInteger);
double? someDouble = null;
Console.WriteLine("This is the real number with Null value -> " +
someDouble);
someDouble = 2.5;
Console.WriteLine("This is the real number with value 5 -> " +
someDouble);
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Variables > Literals
 Primitive types, which we already met, are special data types
built into the C# language.
 Their values specified in the source code of the program are
called literals.
 types of literals:
 Boolean e.g bool open = true;
 Integer
 Real
 Character
 String
 Object literal null
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Variables > Literals > Escaping Sequences
 the most frequently used escaping sequences:
 ' – single quote
 " – double quotes
 – backslash
 n – new line
 t – offset (tab)
 uXXXX – char specified by its Unicode number, for example u1200.
 Example
 char symbol = 'a'; // An ordinary symbol
 symbol = 'u1200'; // Unicode symbol code in a hexadecimal format
 symbol = 'u1201'; // ሁ（”Hu” Amharic Letter）
 symbol = 't'; // Assigning TAB symbol
 symbol = 'n'; // Assigning new line symbol
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Variables > String Literals
 Strings can be preceded by the @ character that specifies a
quoted string (verbatim string)
 string quotation = ""Hello, C#", he said.";
 string path = "C:WindowsNotepad.exe";
 string verbatim = @"The is not escaped as .
I am at a new line.";
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Variables > Scope
 scope
 Visibility/accessibility of a variable
 determines what codes has access to it
 Code block scope
 method scope
 class scope
 Access Modifier / Accessibility/life time extender
 Private – only in the same class
 Internal – in the same assembly
 Protected – same class or subclass (inherited class)
 public – by any object in anywhere
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Type Casting
 Casting – converting one data from one data type to another
 Two type casting
 Implicit casting
> Performed automatically
> Widening conversion
 Used to convert data with a less precise type to a more precise type
> byte-> short -> int -> long->decimal
> E.g.
 double mark =85;
 int grade =‘A’
 Explicit casting
> Narrowing conversion
 Casts data from a more precise data type to a less precise data type
> (type) expression
> E.g
 int mark = (int)85.25;
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Convert data type
 .Net framework provides structures and classes that defines data type
 Structure defines a value type
 Class defines a reference type
 Structure
 Byte – byte – an 8-bit unsigned integer
 Int16 – short - a 16-bit signed integer
 Int32 – int – A 32-bit signed integer
 Int64 – long – A 64-bit signed integer
 Single – float – a single-precision floating number
 Double – double - a double precision floating number
 Decimal – decimal – a 96-bit decimal value
 Boolean – bool – a true or false value
 Char – char - a single character
 Class
 String – string – a reference to a String object
 Object – object – a reference to any type of object
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Convert data type …
 Methods used to convert data types
 ToString
> Converts the value to its equivalent string representation using the specified format, if any
> ToString([format])
 Parse
> A static method that converts the specified string to an equivalent data value
> If the string can’t converted, an exception occurred
> Parse(string)
 TryParse
> A static method that converts the specified string to an equivalent data value and
> Stores it in the result variable
> Returns a true value if the string is converted, otherwise, returns a false value
> If the string can’t converted, an exception occurred
 Static methods of Convert class
> ToDecima(value) – converts the value to the decimal data type
> ToDouble(value) - converts the value to the double data type
> ToInt32(value) – converts the value to the int data type
> …
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Convert data type > Example
decimal salary = 2453.32m;
string salaryString = salary.ToString();
salary=Decimal.Parse(salaryString);
Decimal.TryParse(salaryString, out salary);
double cgpa = 3.85;
string studentCgpa = “CGPA: “+ cgpa; //automatic call to ToString method
string salesString = “$45268.25”;
decimal sales = 0m;
Decimal.TryParse(salesString, out sales); //sales is 0
string gradePoint = “63.25”;
string totalCredit = “22”;
double gpa = Convert.ToDouble(pgradePoint)/Convert.ToInt32(totalCredit);
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Convert data type > Formatted String
 using methods to convert formatted strings
 C or c – Currency
 P or p – Percent
 N or n – Number
 F or f – Float
 D or d – Digits
 E or e – Exponential
 G or g – General
 Use
 ToString method or
 Format method
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Exrcise 1
 declare and initialize the following types of data
 int
 float
 double
 string
 decimal
you must use approprate naming convention.
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exrcise 2
 using approprate data type initialize the lamp status
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Convert data type > Formatted String > Example
 decimal amount = 15632.20m;
 decimal interest = 0.023m;
 int quantity = 15000;
 float payment = 42355.5f;
 ToString method
 string monthlyAmount = amount.ToString(“c”); // $15,632.20
 string interestRate = interest.ToString(“p”); //2.3%
 string quantityString = quantity.ToString(“n”); // 15,000
 string paymentString = payment.ToString(“f”); //42,355.500
 Format method of String Class
 string monthlyAmount = String.Format(“{0:c}”, amount); // $15,632.20
 string interestRate = String.Format(“{0:p}”, interest); //2.3%
 string quantityString = String.Format(“{0:n}”, quantity); // 15,000
 string paymentString = String.Format(“{0:f}”, payment); //42,355.500
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Comments in C#
 To include information about your code
 Not readable by the compiler
 Single line comment
//comment
 Multiple line comment
/* --------------
---- multiline comment -------
--------------*/
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Exercise
1. Which of the ff a valid C# variable name?
A. new
B. 1stYear
C. grade
D. father-name
2. Declare a variable that stores
a. letter grade of a subject
b. CGPA of a student
c. Balance of a customer
3. Assign an initial value for the above variables
a. To their domain default
b. Using new key word
4. What is Nullable data type and its advantage? Give example.
5. Write the output of the ff code snippet
string exercise = @“Please Students answer this exercise .
Please participate actively.";
Console.WriteLine(exercise );
6. Give an example for implicit and explicit type casting.
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C# > Operators and Expressions
Performing Simple Calculations with C#
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Operators and Expressions
 Operator
 is an operation performed over data at runtime
 Takes one or more arguments (operands)
 Produces a new value
 Operators have precedence
 Precedence defines which will be evaluated first
 Expressions
 are sequences of operators and operands that are evaluated to a single
value
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Operators in C#
 Operators in C# :
 Unary – take one operand
 Binary – take two operands
 Ternary – takes three operands
 Except for the assignment operators, all binary operators are left-
associative
 The assignment operators and the conditional operator (?:) and
?? are right-associative
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Operators in C# > Categories of Operators
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Category Operators
Arithmetic +, -, *, /, %, ++, --
Logical &&, ||, ^, !,
Binary/Bitwise &, |, ^, ~, <<, >>
Comparison/Relationa ==, !=, < >, <=, >=
Assignment
=, +=, -=, *=, /=, %=, &=, |=, ^=,
<<=, >>=
String concatenation +
Type conversion is, as, typeof
Other ., [], (), ?:, ??, new

Operators in C# > Operators Precedence
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Precedence Operators
Highest ++ -- (postfix) new typeof
++ -- (prefix) + - (unary) ! ~
* / %
+ -
<< >>
< > <= >= is as
== !=
&
Lower ^

Operators in C# > Operators Precedence
 Parenthesis is operator always has highest precedence
 Note: prefer using parentheses, even when it seems difficult to
do so
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Precedence Operators
Higher |
&&
||
?:, ??
Lowest
=, *=, /=, %=, +=, -=, <<=, >>=, &=, ^=,
|=

Arithmetic Operators
 Arithmetic operators +, -, * are the same as in math
 Division operator / if used on integers returns integer (without
rounding) or exception
 Division operator / if used on real numbers returns real number
or Infinity or NaN
 Remainder operator % returns the remainder from division of
integers
 The special addition operator ++ increments a variable
 The special subtraction operator -- decrements a variable
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Arithmetic Operators > Example
 int squarePerimeter = 17;
 double squareSide = squarePerimeter/4.0;
 double squareArea = squareSide*squareSide;
 Console.WriteLine(squareSide); // 4.25
 Console.WriteLine(squareArea); // 18.0625
 int a = 5;
 int b = 4;
 Console.WriteLine( a + b ); // 9
 Console.WriteLine( a + b++ ); // 9
 Console.WriteLine( a + (++b) ); // 11
 Console.WriteLine(11 / 3); // 3
 Console.WriteLine(11 % 3); // 2
 Console.WriteLine(12 / 3); // 4
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Logical Operator
 Logical operators take boolean operands and return boolean
result
 Operator ! (logical Negation) turns true to false and false to true
 Behavior of the operators &&(AND), ||(OR) and ^ (exclusive OR)
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Operation || || || || && && && && ^ ^ ^ ^
Operand1 0 0 1 1 0 0 1 1 0 0 1 1
Operand2 0 1 0 1 0 1 0 1 0 1 0 1
Result 0 1 1 1 0 0 0 1 0 1 1 0

Logical Operator > Example
 Example
 bool a = true;
 bool b = false;
 Console.WriteLine(a && b); // False
 Console.WriteLine(a || b); // True
 Console.WriteLine(a ^ b); // True
 Console.WriteLine(!b); // True
 Console.WriteLine(b || true); // True
 Console.WriteLine(b && true); // False
 Console.WriteLine(a || true); // True
 Console.WriteLine(a && true); // True
 Console.WriteLine(!a); // False
 Console.WriteLine((5>7) ^ (a==b)); // False
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Bitwise Operators
 Bitwise operator ~ turns all 0 to 1 and all 1 to 0
 Like ! for boolean expressions but bit by bit
 The operators |, & and ^ behave like ||, && and ^ for boolean expressions but bit by bit
 The << and >> move the bits (left or right)
 Bitwise operators are used on integer numbers (byte, sbyte, int, uint, long, ulong)
 Bitwise operators are applied bit by bit
 Behavior of the operators|, & and ^:
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Operation | | | | & & & & ^ ^ ^ ^
Operand1 0 0 1 1 0 0 1 1 0 0 1 1
Operand2 0 1 0 1 0 1 0 1 0 1 0 1
Result 0 1 1 1 0 0 0 1 0 1 1 0

Bitwise Operators > Example
 Examples:
ushort a = 3; // 00000000 00000011
ushort b = 5; // 00000000 00000101
Console.WriteLine( a | b); // 00000000 00000111
Console.WriteLine( a & b); // 00000000 00000001
Console.WriteLine( a ^ b); // 00000000 00000110
Console.WriteLine(~a & b); // 00000000 00000100
Console.WriteLine( a<<1 ); // 00000000 00000110
Console.WriteLine( a>>1 ); // 00000000 00000001
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Comparison Operators
 Comparison operators are used to compare variables
 ==, <, >, >=, <=, !=
 Comparison operators example:
int a = 5;
int b = 4;
Console.WriteLine(a >= b); // True
Console.WriteLine(a != b); // True
Console.WriteLine(a == b); // False
Console.WriteLine(a == a); // True
Console.WriteLine(a != ++b); // False
Console.WriteLine(a > b); // False
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Assignment Operators
 Assignment operators are used to assign a value to a variable ,
 =, +=, -=, |=, ...
 Assignment operators example:
int x = 6;
int y = 4;
Console.WriteLine(y *= 2); // 8
int z = y = 3; // y=3 and z=3
Console.WriteLine(z); // 3
Console.WriteLine(x |= 1); // 7 (0110 | 0001)
Console.WriteLine(x += 3); // 10
Console.WriteLine(x /= 2); // 5
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Other Operators
 String concatenation operator + is used to concatenate strings
 If the second operand is not a string, it is converted to string
automatically
 example
string name= “Name";
string fatherName= “Father Name";
Console.WriteLine(name + fatherName);
// Name FatherName
string output = "The number is : ";
int number = 5;
Console.WriteLine(output + number);
// The number is : 5
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Other Operators …
 Member access operator . is used to access object members
 Square brackets [] are used with arrays, indexers and attributes
 Parentheses ( ) are used to override the default operator precedence
 Class cast operator (type) is used to cast one compatible type to another
 Conditional operator ?: has the form
 b?x:y
> if b is true then the result is x else the result is y
 Null coalescing/joining operator ??
 x=y??0;
> If y is null set x=0 else set x=y
 The new operator is used to create new objects
 The typeof operator returns System.Type object (the reflection of a type)
 The is operator checks if an object is compatible with given type
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Other Operators > Example
 Examples
int a = 6;
int b = 4;
Console.WriteLine(a > b ? "a>b" : "b>=a"); // a>b
Console.WriteLine((long) a); // 6
int c = b = 3; // b=3; followed by c=3;
Console.WriteLine(c); // 3
Console.WriteLine(a is int); // True
Console.WriteLine((a+b)/2); // 4
Console.WriteLine(typeof(int)); // System.Int32
int d = new int();
Console.WriteLine(d); // 0
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Expressions
 Expressions are sequences of operators, literals and variables that are
evaluated to some value
 Expressions has:
 Type (integer, real, boolean, ...)
 Value
 Examples:
int r = (150-20) / 2 + 5; // r=70
// Expression for calculation of circle area
double surface = Math.PI * r * r;
// Expression for calculation of circle perimeter
double perimeter = 2 * Math.PI * r;
int a = 2 + 3; // a = 5
int b = (a+3) * (a-4) + (2*a + 7) / 4; // b = 12
bool greater = (a > b) || ((a == 0) && (b == 0));
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Using the Math Class
 Math class provides methods to work with numeral data
 Math.Round(decimalnumber[, precision, mode]);
 Math.Pow(number, power);
 Math.Sqrt(number);
 Math.Min(number1, number2);
 Math.Max(number1, number2);
 Example
double gpa = Math.Round(gpa,2);
double area = Math.Pow(radius, 2) * Math.PI; //area of a circle
double maxGpa = Math.Max(lastYearGpa, thisYearGpa);
double sqrtX = Math.Sqrt(x);
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Exercise
1. Write conditional expression (Ternary expression) that checks if given
integer is odd or even.
2. Write the output of the ff code fragment.
sbyte x = 9;
Console.WriteLine( "Shift of {0} is {1}.", x, ~x<<3);
ushort a = 3;
ushort b = 5;
Console.WriteLine(a ^ ~b);
int a = 3;
int b = 5;
Console.WriteLine(“a + b = ” + a + b);
5. Write a program that calculates the area of a circle for a given radius r
(eqn: a= 𝜋𝑟2
).
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Console Input / Output
Reading and Writing to the Console
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Console Input / Output
 Printing to the Console
 Printing Strings and Numbers
 Reading from the Console
 Reading Characters
 Reading Strings
 Parsing Strings to Numeral Types
 Reading Numeral Types
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Printing to the Console
 Console is used to display information in a text window
 Can display different values:
 Strings
 Numeral types
 All primitive data types
 To print to the console use the namespace system and class
Console (System.Console)
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The Console Class
 Provides methods for console input and output
 Input
> Read(…) – reads a single character
> ReadKey(…) – reads a combination of keys
> ReadLine(…) – reads a single line of characters
 Output
> Write(…) – prints the specified argument on the console
> WriteLine(…) – prints specified data to the console and moves to the next line
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Console.Write(…)
 Printing an integer variable
int a = 15;
...
Console.Write(a); // 15
 Printing more than one variable using a formatting string
double a = 15.5;
int b = 14;
...
Console.Write("{0} + {1} = {2}", a, b, a + b);
// 15.5 + 14 = 29.5
 Next print operation will start from the same line
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Console.WriteLine(…)
 Printing a string variable
string str = "Hello C#!";
...
Console.WriteLine(str);
 Printing more than one variable using a formatting string
string name = “Fatuma";
int year = 1990;
...
Console.WriteLine("{0} was born in {1}.", name, year);
// Fatuma was born in 1990.
 Next printing will start from the next line
4/13/2021 58

Reading from the Console
 Reading Strings and Numeral Types
 We use the console to read information from the command line
 We can read:
 Characters
 Strings
 Numeral types (after conversion)
 To read from the console we use the methods
 Console.Read() and
 Console.ReadLine()
4/13/2021 59

Console.Read()
 Gets a single character from the console (after [Enter] is pressed)
 Returns a result of type int
 Returns -1 if there aren’t more symbols
 To get the actually read character we
need to cast it to char
int i = Console.Read();
char ch = (char) i; // Cast the int to char
// Gets the code of the entered symbol
Console.WriteLine("The code of '{0}' is {1}.", ch, i);
4/13/2021 60

Console.ReadKey()
 Waits until a combination of keys is pressed
 Reads a single character from console or a combination of keys
 Returns a result of type ConsoleKeyInfo
 KeyChar – holds the entered character
 Modifiers – holds the state of [Ctrl], [Alt], …
ConsoleKeyInfo key = Console.ReadKey();
Console.WriteLine();
Console.WriteLine("Character entered: " + key.KeyChar);
Console.WriteLine("Special keys: " + key.Modifiers);
4/13/2021 61

Console.ReadLine()
 Gets a line of characters
 Returns a string value
 Returns null if the end of the input is reached
Console.Write("Please enter your first name: ");
string firstName = Console.ReadLine();
Console.Write("Please enter your last name: ");
string lastName = Console.ReadLine();
Console.WriteLine("Hello, {0} {1}!", firstName, lastName);
4/13/2021 62

Reading Numeral Types
Numeral types can not be read directly from the console
To read a numeral type do the following:
1. Read a string value
2. Convert (parse) it to the required numeral type
int.Parse(string) – parses a string to int
string str = Console.ReadLine()
int number = int.Parse(str);
Console.WriteLine("You entered: {0}", number);
string s = "123";
int i = int.Parse(s); // i = 123
long l = long.Parse(s); // l = 123L
string invalid = "xxx1845";
int value = int.Parse(invalid); // FormatException
4/13/2021 63

Reading Numbers from the Console > Example
static void Main()
{
int a = int.Parse(Console.ReadLine());
int b = int.Parse(Console.ReadLine());
Console.WriteLine("{0} + {1} = {2}", a, b, a+b);
Console.WriteLine ("{0} * {1} = {2}", a, b, a*b);
float f = float.Parse(Console.ReadLine());
Console.WriteLine("{0} * {1} / {2} = {3}", a, b, f, a*b/f);
}
4/13/2021 64

Error Handling when Parsing
 Sometimes we want to handle the errors when parsing a number
 Two options: use TryParse() or try-catch block (later in Error Handling
Section)
 Parsing with TryParse():
string str = Console.ReadLine();
int number;
if (int.TryParse(str, out number)) // out is akey word
{
Console.WriteLine("Valid number: {0}", number);
}
else
{
Console.WriteLine("Invalid number: {0}", str);
}
4/13/2021 65

Console IO > Example
 Calculating an Area
Console.WriteLine("This program calculates the area of a rectangle or a
triangle");
Console.Write("Enter a and b (for rectangle) or a and h (for triangle): ");
//read inputs
int a = int.Parse(Console.ReadLine());
int b = int.Parse(Console.ReadLine());
Console.Write("Enter 1 for a rectangle or 2 for a triangle: ");
int choice = int.Parse(Console.ReadLine());
//calculate the area
double area = (double) (a*b) / choice;
//display the result
Console.WriteLine("The area of your figure is {0}", area);
4/13/2021 66

Conditional Statements
Implementing Control Logic in C#
4/13/2021 67

Conditional Statements
 Implementing Control Logic in C#
 if Statement
 if-else Statement
 nested if Statements
 multiple if-else-if-else-…
 switch-case Statement
4/13/2021 68

The if Statement
 The most simple conditional statement
 Enables you to test for a condition
 Branch to different parts of the code depending on the result
 The simplest form of an if statement:
if (condition)
{
statements;
}
4/13/2021 69
true
condition
statement
false

The if Statement > Example
static void Main()
{
Console.WriteLine("Enter two numbers.");
int biggerNumber = int.Parse(Console.ReadLine());
int smallerNumber = int.Parse(Console.ReadLine());
if (smallerNumber > biggerNumber)
{
biggerNumber = smallerNumber;
}
Console.WriteLine("The greater number is: {0}", biggerNumber);
}
4/13/2021 70

The if-else Statement
 More complex and useful conditional statement
 Executes one branch if the condition is true, and another if it is
false
 The simplest form of an if-else statement:
if (condition)
{
statement1;
}
else
{
statement2;
}
4/13/2021 71
condition
first
statement
true
second
statement
false

if-else Statement > Example
 Checking a number if it is odd or even
string s = Console.ReadLine();
int number = int.Parse(s);
if (number % 2 == 0)
{
Console.WriteLine("This number is even.");
}
else
{
Console.WriteLine("This number is odd.");
}
4/13/2021 72

Nested if Statements
 if and if-else statements can be nested, i.e. used inside another if or
else statement
 Every else corresponds to its closest preceding if
if (expression)
{
if (expression)
{
statement;
}
else
{
statement;
}
}
else
statement;
4/13/2021 73

Nested if Statements > Example
if (first == second)
{
Console.WriteLine("These two numbers are equal.");
}
else
{
if (first > second)
{
Console.WriteLine("The first number is bigger.");
}
else
{
Console.WriteLine("The second is bigger.");
}
}
4/13/2021 74

Multiple if-else-if-else-…
 Sometimes we need to use another if-construction in the else
block
 Thus else if can be used:
int ch = 'X';
if (ch == 'A' || ch == 'a')
{
Console.WriteLine("Vowel [ei]");
}
else if (ch == 'E' || ch == 'e')
{
Console.WriteLine("Vowel [i:]");
}
else if …
else …
4/13/2021 75

The switch-case Statement
 Selects for execution a statement from a list depending on the value
of the switch expression
switch (day)
{
case 1: Console.WriteLine("Monday"); break;
case 2: Console.WriteLine("Tuesday"); break;
case 3: Console.WriteLine("Wednesday"); break;
case 4: Console.WriteLine("Thursday"); break;
case 5: Console.WriteLine("Friday"); break;
case 6: Console.WriteLine("Saturday"); break;
case 7: Console.WriteLine("Sunday"); break;
default: Console.WriteLine("Error!"); break;
}
4/13/2021 76

How switch-case Works?
1. The expression is evaluated
2. When one of the constants specified in a case label is equal to
the expression
 The statement that corresponds to that case is executed
3. If no case is equal to the expression
 If there is default case, it is executed
 Otherwise the control is transferred to the end point of the switch
statement
4/13/2021 77

switch-case > Usage good practice
 Variables types like string, enum and integral types can be used for switch
expression
 The value null is permitted as a case label constant
 The keyword break exits the switch statement
 "No fall through" rule – you are obligated to use break after each case
 Multiple labels that correspond to the same statement are permitted
 There must be a separate case for every normal situation
 Put the normal case first
 Put the most frequently executed cases first and the least frequently executed last
 Order cases alphabetically or numerically
 In default use case that cannot be reached under normal circumstances
4/13/2021 78

Multiple Labels – Example
 You can use multiple labels to execute the same statement in more than one
case
switch (animal)
{
case "dog" :
Console.WriteLine("MAMMAL");
break;
case "crocodile" :
case "tortoise" :
case "snake" :
Console.WriteLine("REPTILE");
break;
default :
Console.WriteLine("There is no such animal!");
break;
}
4/13/2021 79

Looping / Iterations Statements
Execute Blocks of Code Multiple Times
4/13/2021 80

Looping Statements
 A loop is a control statement that allows repeating execution of a
block of statements
 May execute a code block fixed number of times
 May execute a code block while given condition holds
 May execute a code block for each member of a collection
 Loops that never end are called an infinite loops
 Loops in C#
 while loops
 do … while loops
 for loops
 foreach loops
 Nested loops
4/13/2021 81

Using while(…) Loop
 Repeating a Statement While Given Condition Holds
 The simplest and most frequently used loop
while (condition)
{
statements;
}
 The repeat condition
 Returns a boolean result of true or false
 Also called loop condition
4/13/2021 82
true
condition
statement
false

While Loop > Example
int counter = 0;
while (counter < 10)
{
Console.WriteLine("Number : {0}", counter);
counter++;
}
4/13/2021 83

While Loop > Example
 Checking whether a number is prime or not
Console.Write("Enter a positive integer number: ");
uint number = uint.Parse(Console.ReadLine());
uint divider = 2;
uint maxDivider = (uint) Math.Sqrt(number);
bool prime = true;
while (prime && (divider <= maxDivider))
{
if (number % divider == 0)
{
prime = false;
}
divider++;
}
Console.WriteLine("Prime? {0}", prime);
4/13/2021 84

Do-While Loop
 Another loop structure is:
do
{
statements;
}
while (condition);
 The block of statements is repeated
 While the boolean loop condition holds
 The loop is executed at least once
4/13/2021 85
true
condition
statement
false

Do … while > Example
 Calculating N factorial
static void Main()
{
int n = Convert.ToInt32(Console.ReadLine());
int factorial = 1;
do
{
factorial *= n;
n--;
}
while (n > 0);
Console.WriteLine("n! = " + factorial);
}
4/13/2021 86

for loops
 The typical for loop syntax is:
for (initialization; test; update)
{
statements;
}
 Consists of
 Initialization statement
> Executed once, just before the loop is entered
 Boolean test expression
> Evaluated before each iteration of the loop
 If true, the loop body is executed
 If false, the loop body is skipped
 Update statement
> Executed at each iteration after the body of the loop is finished
 Loop body block
4/13/2021 87

for Loop > Example
 A simple for-loop to print the numbers 0…9:
for (int number = 0; number < 10; number++)
{
Console.Write(number + " ");
}
 A simple for-loop to calculate n!:
decimal factorial = 1;
for (int i = 1; i <= n; i++)
{
factorial *= i;
}
4/13/2021 88

foreach Loop
 Iteration over a Collection
 The typical foreach loop syntax is:
foreach (Type element in collection)
{
statements;
}
 Iterates over all elements of a collection
 The element is the loop variable that takes sequentially all collection
values
 The collection can be list, array or other group of elements of the same
type
4/13/2021 89

foreach Loop > Example
 Example of foreach loop:
string[] days = new string[] {
"Monday", "Tuesday", "Wednesday", "Thursday",
"Friday", "Saturday", "Sunday" };
foreach (string day in days)
{
Console.WriteLine(day);
}
 The above loop iterates of the array of days
 The variable day takes all its values
4/13/2021 90

Nested Loops
 A composition of loops is called a nested loop
 A loop inside another loop
 Example:
{
{
statements;
}
…
}
4/13/2021 91

Nested loop > Example
 Print the following triangle:
1
1 2
…
1 2 3 ... n
int n = int.Parse(Console.ReadLine());
for(int row = 1; row <= n; row++)
{
for(int column = 1; column <= row; column++)
{
Console.Write("{0} ", column);
}
}
4/13/2021 92

C# Jump Statements
Jump statements are:
break, continue, goto, return
How continue woks?
In while and do-while loops jumps to the test expression
In for loops jumps to the update expression
To exit an inner loop use break
To exit outer loops use goto with a label
 Avoid using goto! (it is considered harmful)
return – to terminate method execution and go back to the caller
method
4/13/2021 93

Jump Statements > Example
int outerCounter = 0;
for (int outer = 0; outer < 10; outer++)
{
for (int inner = 0; inner < 10; inner++)
{
if (inner % 3 == 0)
continue;
if (outer == 7)
break;
if (inner + outer > 9)
goto breakOut;
}
outerCounter++;
}
breakOut:
4/13/2021 94

Jump Statements > continue example
 Example: sum all odd numbers in [1, n] that are not divisors of 7:
int sum = 0;
for (int i = 1; i <= n; i += 2)
{
if (i % 7 == 0)
{
continue;
}
sum += i;
}
Console.WriteLine("sum = {0}", sum);
4/13/2021 95

For more information
 Brian Bagnall, et al. C# for Java Programmers. USA. Syngress
Publishing, Inc.
 http://www.tutorialspoint.com/csharp/
 Svetlin Nakov et al. Fundamentals of Computer Programming
With C#. Sofia, 2013
 Joel Murach, Anne Boehm. Murach C# 2012, Mike Murach &
Associates Inc USA, 2013
4/13/2021 96

Date and string data types
Working with Date and string data types
4/13/2021 97

Dates and Times
 Data processing on date and time values
 DateTime structure of .Net framework provides a variety of properties and methods for
 Getting information about dates and times
 Formatting date and time values
 Performing operations on dates and times
 To create a DateTime value
 Use new key word
 Specify the date and time values
Syntax:
DateTime variable = new DateTime(year, month, day[, hour, minute, second[, millisecond]]);
 If time is not specified, it set to 12:00 AM.
 Use also static Parse and TryParse method to create DateTime value from a string
DateTime variable = DateTime.Parse(string);
DateTime variable;
DateTime.TryParse(string, out variable);
4/13/2021 98

Dates and Times > Example
DateTime startDate = new DateTime(2019, 04, 30); //
DateTime startDateTime = new DateTime(2019, 01, 30, 13, 30, 0); //
DateTime startDate = DateTime.Parse(“03/08/16”); //
DateTime startDate = DateTime.Parse (“Apr 08, 2019 1:30 PM”); //
DateTime dateOfBirth= DateTime.Parse (Console.ReadLine()); //
DateTime expDate;
DateTime.TryParse (Console.ReadLine(), out expDate); //
DateTime deadlineDate;
Console.WriteLine(“Enter deadline Date (mm/dd/yyyy) :”);
deadlineDate = Convert.ToDateTime(Console.ReadLine());
4/13/2021 99

Dates and Times > Date and Time writing format
 Valid date format includes:
 04/30/2019
 1/30/19
 01-30-2019
 1-30-19
 2019-30-1
 Jan 30 2019
 January 30, 2019
 Valid time format includes:-
 2:15 PM
 14:15
 02:15:30 AM
4/13/2021 100

Dates and Times > Current Date and Time
 Use two static properties of DateTime structure to get the
current date/time
 Now
> Returns the current date and time
 Today
> Returns the current date
 Example
DateTime currDateTime = DateTime.Now; //
DateTime currDate = DateTime.Today; //
DateTime regDateTime = DateTime.Toady;
DateTime modifiedDate = DateTime.Now;
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Dates and Times > Date and Time formatting
 The format depends on the computer regional setting
 Use the methods of DateTime structure to format the date/time in
the way you want
 ToLongDateString()
 Name for the day of the week, the name for the month, the date of the
month, and the year
 ToShortDateString()
 The numbers for the month, day, and year
 ToLongTimeString()
 The hours, minutes and seconds
 ToShortTimeString()
 The hours and minutes
4/13/2021 102

Dates and Times > Date and Time formatting >Example
 Statements that format dates and times data
DateTime currDateTime = DateTime.Now;
string longDate = currDateTime.ToLongDateString(); //
string shortDate = currDateTime.ToShortDateString(); //
string longTime = currDateTime.ToLongTimeString(); //
string shortDate = currDateTime.ToShortTimeString(); //
4/13/2021 103

Dates and Times > Getting information about Dates and Times
 The DateTime structure provides a variety of properties and methods for getting
information about dates and times
4/13/2021 104

Getting information about Dates and Times > Example
DateTime currDateTime = DateTime.Now;
int month = currDateTime.Month;
int hour = currDateTime.Hour;
int dayOfYear = currDateTime.DayOfYear;
int daysInMonth = DateTime.DaysInMonth(currDateTime.Year, 2);
bool isLeapYear = currDateTime.IsLeapYear();
DayOfWeek dayOfWeek = currDateTime.DayOfWeek;
string message=“”:
if(dayOfWeek == DayOfWeek.Saturday || dayOfWeek == DayOfWeek.Sunday ) {
message = “Weekend”;
}
else {
message = “Week day”;
}
4/13/2021 105

Dates and Times > Perform Operations on Dates and Times
 Methods for performing operations on dates and times
4/13/2021 106

Perform Operations on Dates and Times > Example
DateTime dateTime = DateTime.Parse(“1/3/2016 13:30”);
DateTime dueDate = dateTime.AddMonths(2);
dueDate = dateTime.AddDays(60);
DateTime runTime = dateTime.AddMinutes(30);
runTime = dateTime.AddHours(10);
DateTime currentDate = DateTime.Today;
DateTime deadLine = DateTime.Parse(“3/15/2016”);
TimeSpan timeTillDeadline = deadLine.Subtract(currentDate);
int daysTillDeadline = timeTillDeadline.Days;
TimeSpan timeTillDeadline = deadLine – currentDate;
double totalDaysTillDeadline = timeTillDeadline.TotalDays;
int hoursTillDeadline= timeTillDeadline.Hours;
int minutesTillDeadline= timeTillDeadline.Minutes;
int secondsTillDeadline= timeTillDeadline.Seconds;
bool passedDeadline = false;
if(currentDate > deadLine ){
passedDeadline = true;
}
4/13/2021 107

Format dates and times
 Use Format methods of the String class to format dates and
times
 Standard DateTime formatting
4/13/2021 108

Format dates and times …
 Custom DateTime formatting
4/13/2021 109

Format dates and times > Example
DateTime currDate = DateTime.Now;
string formattedDate = “”;
formattedDate = String.Format(“{0:d}”, currDate);
formattedDate = String.Format(“{0:D}”, currDate);
formattedDate = String.Format(“{0:t}”, currDate);
formattedDate = String.Format(“{0:T}”, currDate);
formattedDate = String.Format(“{0:ddd, MMM d, yyyy}”, currDate);
formattedDate = String.Format(“{0:M/d/yy}”, currDate);
formattedDate = String.Format(“{0:HH:mm:ss}”, currDate);
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Working with Strings
Processing and Manipulating Text Information
4/13/2021 111

Working with Strings
 Working with characters in a string
 Working with string – creating string object from String class
 Use properties and methods of String class to work with string
object
 StringBuilder class also provides another properties and methods
to work with string
4/13/2021 112

Properties and methods of String class
 Common properties and methods of String class
 [index] – gets the character at the specified position
 Length – gets the number of characters
 StartsWith(string)
 EndsWidth(string)
 IndexOf(string[, startIndex])
 LastIndexOf(string[, startIndex])
 Insert(startIndex, string)
 PadLeft(totalWidth)
 PadRight(totalWidth)
 Remove(startIndex, count)
 Replace(oldString, newString)
 Substring(startIndex[, length])
 ToLower()
 ToUpper()
 Trim()
 Split(splitCharacters)
 Use an index to access each character in a string where 0 in the index for the first character, 1 the
index for the second character, …
4/13/2021 113

Using StringBuilder class
 StringBuilder objects are mutable, means they can be changed, deleted, replaced, modified
 Syntax
 StringBuilder var = new StringBuilder([value] [, ] [capacity]); // default capacity is 16 characters
4/13/2021 114

StringBuilder > Example
 StringBuilder address1 = new StringBuilder();
 StringBuilder address2 = new StringBuilder(10);
 StringBuilder phone1= new StringBuilder(“0912345678”);
 StringBuilder phone2 = new StringBuilder(“0912345678”, 10);
 StringBuilder phoneNumber = new StringBuilder(10);
 phoneNumber.Append(“0912345678”);
 phoneNumber.Insert(0, “(+251)”);
 phoneNumber.Insert(4, “.”);
 phoneNumber.Replace(“.”, “-”);
 phoneNumber.Remove(0, 4);
4/13/2021 115

Manipulating Strings
Comparing, Concatenating, Searching, Extracting Substrings, Splitting
4/13/2021 116

Comparing Strings
 A number of ways exist to compare two strings:
 Dictionary-based string comparison
> Case-insensitive
 int result = string.Compare(str1, str2, true);
 // result == 0 if str1 equals str2
 // result < 0 if str1 is before str2
 // result > 0 if str1 is after str2
> Case-sensitive
 string.Compare(str1, str2, false);
 Equality checking by operator ==
 Performs case-sensitive compare
> if (str1 == str2)
> {
> …
> }
 Using the case-sensitive Equals() method
 The same effect like the operator ==
> if (str1.Equals(str2))
> {
> …
> }
4/13/2021 117

Comparing Strings – Example
 Finding the first string in alphabetical order from a given list of strings:
string[] towns = {“Hawassa", “Dilla", “Adama",
“Mekele", “Debre Berhan", “Dessie", “Gonder"};
string firstTown = towns[0];
for (int i=1; i<towns.Length; i++)
{
string currentTown = towns[i];
if (String.Compare(currentTown, firstTown) < 0)
{
firstTown = currentTown;
}
}
Console.WriteLine("First town: {0}", firstTown);
4/13/2021 118

Concatenating Strings
 There are two ways to combine strings:
 Using the Concat() method
> string str = String.Concat(str1, str2);
 Using the + or the += operators
> string str = str1 + str2 + str3;
> string str += str1;
 Any object can be appended to string
> string name = “Lemma";
> int age = 22;
> string s = name + " " + age; // Lemma 22
4/13/2021 119

Concatenating Strings – Example
string firstName = “Soliana";
string lastName = “Abesselom";
string fullName = firstName + " " + lastName;
Console.WriteLine(fullName);// Soliana Abesselom
int age = 25;
string nameAndAge ="Name: " + fullName + "nAge: " + age;
Console.WriteLine(nameAndAge);
// Name: Soliana Abesselom
// Age: 25
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Searching in Strings
 Finding a character or substring within given string
 First occurrence
> IndexOf(string str);
 First occurrence starting at given position
> IndexOf(string str, int startIndex)
 Last occurrence
> LastIndexOf(string)
 IndexOf is case-sensetive
4/13/2021 121

Searching in Strings > Example
 string str = "C# Programming Course";
 int index = str.IndexOf("C#");
 index = str.IndexOf("Course");
 index = str.IndexOf("COURSE");
 index = str.IndexOf("ram");
 index = str.IndexOf("r");
 index = str.IndexOf("r", 5);
 index = str.IndexOf("r", 8);
4/13/2021 122

Extracting Substrings
 Extracting substrings
 str.Substring(int startIndex, int length)
> string filename = @"C:Picsbird2009.jpg";
> string name = filename.Substring(8, 8);
 str.Substring(int startIndex)
> string filename = @"C:PicsSummer2009.jpg";
> string nameAndExtension = filename.Substring(8);
> // nameAndExtension is Summer2009.jpg
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Splitting Strings
 To split a string by given separator(s) use the following method:
 string[] Split(params char[])
string listOfBeers = “Bedelle, Habesha Raya, Dashen Giorgis, Meta";
string[] beers = listOfBeers.Split(' ', ',', '.');
Console.WriteLine("Available beers are:");
foreach (string beer in beers)
{
Console.WriteLine(beer);
}
4/13/2021 124

Trimming White Space
 Using method Trim()
 string s = " example of white space ";
 string clean = s.Trim();
 Console.WriteLine(clean);
 Using method Trim(chars)
 string s = " tnHello!!! n";
 string clean = s.Trim(' ', ',' ,'!', 'n','t');
 Console.WriteLine(clean); //
 Using TrimStart() and TrimEnd()
 string s = " C# ";
 string clean = s.TrimStart(); // clean =
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Methods, events and delegates
Subroutines in Computer Programming
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Methods
 A method is a kind of building block that solves a small problem
 A piece of code that has a name and can be called from the other code
 Can take parameters and return a value
 Can be public or private
 Methods allow programmers to construct large programs from
simple pieces
 Methods are also known as functions, procedures, and
subroutines
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Why to Use Methods?
 More manageable programming
 Split large problems into small pieces
 Better organization of the program
 Improve code readability
 Improve code understandability
 Avoiding repeating code
 Improve code maintainability
 Code reusability
 Using existing methods several times
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Declaring and Creating methods
 Each Method has
 Name
 Access modifier
 Return type
 Parameters/arguments
 A body /statements
Syntax
access_modifier return_type name(parametrs){
statements;
}
example
public double CalculateGpa(double totalGradePoint, int totalCredit){
return totalGradePoint/totalCredit;
}
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Calling Methods
 To call a method, simply use:
 The method’s name
 Pass value
 Accept return value if any
 Example
 double cgpa = calculateGpa(92.23, 36);
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Optional Parameters
 C# 4.0 supports optional parameters with default values:
void PrintNumbers(int start=0; int end=100)
{
for (int i=start; i<=end; i++)
{
Console.Write("{0} ", i);
}
}
 The above method can be called in several ways:
PrintNumbers(5, 10);
PrintNumbers(15);
PrintNumbers();
 If you define an optional parameter, every parameters after that parameters must be defined
as optional
 If you pass a value to an optional parameter by position, you must pass a value to all
parameters before that parameter
 You can also pass value by name, code the parameter name followed by a colon followed by
the value/argument name
PrintNumbers(end: 40, start: 35);
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Passing parameters by value and reference
 When calling a method, each argument can be passed by value or by reference
 Pass by value
 Original value will not be changed by the calling method
 Pass by reference
 Original value can be changed by the calling method
 to pass by reference use
> ref or
> out keyword
 No need to initialize the argument, assign value to it within the calling method
 Example
void PrintSum(int start; int end, int ref sum)
{
for (int i=start; i<=end; i++)
{
sum+=I;
}
}
//
int sum =0;
PrintSum(start, end, ref sum);
Console.WriteLine(“Sum {0}”, sum);
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Recursive methods
 The Power of Calling a Method from Itself
 Example
static decimal Factorial(decimal num)
{
if (num == 0)
return 1;
else
return num * Factorial(num - 1);
}
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Events, delegates and Indexer
 Events are user actions such as key press, clicks, mouse movements,
etc., or some occurrence such as system generated notifications.
 Applications need to respond to events when they occur.
 For example, interrupts.
 Events are used for inter-process communication.
 A delegate is a reference type variable that holds the reference to a
method.
 The reference can be changed at runtime.
 An indexer allows an object to be indexed such as an array.
 When you define an indexer for a class, this class behaves similar to a
virtual array.
 Reading assignment about Events, delegates and Indexer
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Object Oriented Programming
Modeling Real-world Entities with Objects
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Objects
 Software objects model real-world objects or abstract concepts
 Examples:
> bank, account, customer, dog, bicycle, queue
 Real-world objects have states and behaviors
 Account' states:
> holder, balance, type
 Account' behaviors:
> withdraw, deposit, suspend
 How do software objects implement real-world objects?
 Use variables/data to implement states
 Use methods/functions to implement behaviors
 An object is a software bundle of variables and related methods
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Class
 Classes act as templates from which an instance of an object is
created at run time.
 Classes define the properties of the object and the methods used
to control the object's behavior.
 By default the class definition encapsulates, or hides, the data
inside it.
 Key concept of object oriented programming.
 The outside world can see and use the data only by calling the
build-in functions; called “methods”
 Methods and variables declared inside a class are called
members of that class.
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Objects vs Class
 An instance of a class is called an object.
 Classes provide the structure for objects
 Define their prototype, act as template
 Classes define:
 Set of attributes
> Represented by variables and properties
> Hold their state
 Set of actions (behavior)
> Represented by methods
 A class defines the methods and types of data associated with an
object
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Account
+Owner: Person
+Ammount: double
+Suspend()
+Deposit(sum:double)
+Withdraw(sum:double)

Classes in C#
 An object is a concrete instance of a particular class
 Creating an object from a class is called instantiation
 Objects have state
 Set of values associated to their attributes
 Example:
 Class: Account
 Objects: Abebe's account, Kebede's account
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Classes in C#
 Basic units that compose programs
 Implementation is encapsulated (hidden)
 Classes in C# can contain:
 Access Modifiers
 Fields (member variables)
 Properties
 Methods
 Constructors
 Inner types
 Etc. (events, indexers, operators, …)
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Classes in C#
 Classes in C# could have following members:
 Fields, constants, methods, properties, indexers, events, operators,
constructors, destructors
 Inner types (inner classes, structures, interfaces, delegates, ...)
 Members can have access modifiers (scope)
 public, private, protected, internal
 Members can be
 static (common) or specific for a given object
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Classes in C# – Examples
 Example of classes:
 System.Console
 System.String (string in C#)
 System.Int32 (int in C#)
 System.Array
 System.Math
 System.Random
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Simple Class Definition
public class Cat {
private string name;
private string owner;
public Cat(string name, string owner)
{
this.name = name;
this.owner = owner;
}
public string Name
{
get { return name; }
set { name = value; }
}
public string Owner
{
get { return owner;}
set { owner = value; }
}
public void SayMiau()
{
Console.WriteLine("Miauuuuuuu!");
}
}
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Access Modifiers
 Class members can have access modifiers
 Used to restrict the classes able to access them
 Supports the OOP principle "encapsulation"
 Class members can be:
 public – accessible from any class
 protected – accessible from the class itself and all its descendent
classes
 private – accessible from the class itself only
 internal – accessible from the current assembly (used by default)
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Fields and Properties
 Fields are data members of a class
 Can be variables and constants
 Accessing a field doesn’t invoke any actions of the object
 Example:
 String.Empty (the "" string)
 Constant fields can be only read
 Variable fields can be read and modified
 Usually properties are used instead of directly accessing variable fields
 // Accessing read-only field
 String empty = String.Empty;
 // Accessing constant field
 int maxInt = Int32.MaxValue;
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Properties
 Properties look like fields (have name and type), but they can contain code,
executed when they are accessed
 Usually used to control access to data
fields (wrappers), but can contain more complex logic
 Can have two components (and at least one of them) called accessors
 get for reading their value
 set for changing their value
 According to the implemented accessors properties can be:
 Read-only (get accessor only)
 Read and write (both get and set accessors)
 Write-only (set accessor only)
 Example of read-only property:
 String.Length
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The Role of Properties
 Expose object's data to the outside world
 Control how the data is manipulated
 Properties can be:
 Read-only
 Write-only
 Read and write
 Give good level of abstraction
 Make writing code easier
 Properties should have:
 Access modifier (public, protected, etc.)
 Return type
 Unique name
 Get and / or Set part
 Can contain code processing data in specific way
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Defining Properties – Example
public class Point
{
private int xCoord;
private int yCoord;
public int XCoord
{
get { return xCoord; }
set { xCoord = value; }
}
public int YCoord
{
get { return yCoord; }
set { yCoord = value; }
}
// More code ...
}
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Instance and Static Members
 Fields, properties and methods can be:
 Instance (or object members)
 Static (or class members)
 Instance members are specific for each object
 Example: different dogs have different name
 Static members are common for all instances of a class
 Example: DateTime.MinValue is shared between all instances of
DateTime
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Instance and Static Members – Examples
Example of instance member
 String.Length
> Each string object has different length
Example of static member
 Console.ReadLine()
> The console is only one (global for the program)
> Reading from the console does not require to create an instance of it
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Static vs. Non-Static
 Static:
 Associated with a type, not with an instance
 Non-Static:
 The opposite, associated with an instance
 Static:
 Initialized just before the type is used for the first time
 Non-Static:
 Initialized when the constructor is called
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Methods
 Methods manipulate the data of the object to which they belong
or perform other tasks
 Examples:
Console.WriteLine(…)
Console.ReadLine()
String.Substring(index, length)
Array.GetLength(index)
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Static Methods
 Static methods are common for all instances of a class (shared
between all instances)
 Returned value depends only on the passed parameters
 No particular class instance is available
 Syntax:
 The name of the class, followed by the name of the method, separated
by dot
 <class_name>.<method_name>(<parameters>)
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Interface vs. Implementation
 The public definitions comprise the interface for the class
 A contract between the creator of the class and the users of the class.
 Should never change.
 Implementation is private
 Users cannot see.
 Users cannot have dependencies.
 Can be changed without affecting users.
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Constructors
 is a method with the same name as the class.
 It is invoked when we call new to create an instance of a class.
 In C#, unlike C++, you must call new to create an object.
 Just declaring a variable of a class type does not create an object.
 Example
class Student{
private string name;
private fatherName;
public Student(string n, string fn){
name = n;
fatherName = fn;
}
}
 If you don’t write a constructor for a class, the compiler creates a default
constructor.
 The default constructor is public and has no arguments.
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Multiple Constructors
 A class can have any number of constructors.
 All must have different signatures.
 The pattern of types used as arguments
 This is called overloading a method.
 Applies to all methods in C#.
 Not just constructors.
 Different names for arguments don’t matter, Only the types.
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Structures
 Structures are similar to classes
 Structures are usually used for storing data structures, without
any other functionality
 Structures can have fields, properties, etc.
 Using methods is not recommended
 Structures are value types, and classes are reference types
Example of structure
 System.DateTime – represents a date and time
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Namespaces
 Organizing Classes Logically into Namespaces
 Namespaces are used to organize the source code into more logical and
manageable way
 Namespaces can contain
 Definitions of classes, structures, interfaces and other types and other namespaces
 Namespaces can contain other namespaces
 For example:
 System namespace contains Data namespace
 The name of the nested namespace is System.Data
 A full name of a class is the name of the class preceded by the name of its
namespace
 Example:
 Array class, defined in the System namespace
 The full name of the class is System.Array
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Including Namespaces
 The using directive in C#:
 using <namespace_name>
 Allows using types in a namespace, without specifying their full
name
 Example:
 using System;
 DateTime date;
 instead of
 System.DateTime date;
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Generic Classes
 Parameterized Classes and Methods
 Generics allow defining parameterized classes that process data
of unknown (generic) type
 The class can be instantiated with several different particular types
 Example: List<T>  List<int> / List<string> /
List<Student>
 Generics are also known as "parameterized types" or "template
types"
 Similar to the templates in C++
 Similar to the generics in Java
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Generics – Example
public class GenericList<T>
{
public void Add(T element) { … }
}
class GenericListExample
{
static void Main()
{
// Declare a list of type int
GenericList<int> intList = new GenericList<int>();
// Declare a list of type string
GenericList<string> stringList = new GenericList<string>();
}
}
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Collections
Processing Sequences of Elements and set of elements
4/13/2021 164

Array
 An array is a sequence of elements
 All elements are of the same type
 The order of the elements is fixed
 Has fixed size (Array.Length)
 Zero based index
 Is reference type
 Declaration - defines the type of the elements
 Square brackets [] mean "array"
 Examples:
> int[] myIntArray;
> string[] myStringArray;
 Use the operator new
> Specify array length
> int [] myIntArray = new int[5];
 Creating and initializing can be done together:
> int [] myIntArray = {1, 2, 3, 4, 5}; or int [] myintArray = new int[] {1, 2, 3, 4, 5}; or int [] myintArray = new int[5] {1, 2, 3, 4, 5};
 The new operator is not required when using curly brackets initialization
 C# provides automatic bounds checking for arrays
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C# Array Types
There are 3 types of arrays in C# programming:
 Single Dimensional Array
 Multidimensional Array
 Jagged Array

Creating Array > Example
 Creating an array that contains the names of the days of the
week
string[] daysOfWeek =
{
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday",
"Sunday"
};
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Accessing Array Elements
 Read and Modify Elements by Index
 Array elements are accessed using the square brackets operator
[] (indexer)
 Array indexer takes element’s index as parameter
 The first element has index 0
 The last element has index Length-1
 Array elements can be retrieved and changed by the [] operator
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Accessing Array Elements > Example
 Reading Arrays From the Console
 First, read from the console the length of the array
 Next, create the array of given size and read its elements in a for loop
int[] arr = new int[n];
for (int i=0; i<n; i++)
{
arr[i] = int.Parse(Console.ReadLine());
}
 Print each element to the console
string[] array = {"one", "two", "three"};
// Process all elements of the array
for (int index = 0; index < array.Length; index++)
{
// Print each element on a separate line
Console.WriteLine("element[{0}] = {1}",
index, array[index]);
}
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Accessing Array Elements > Example
 Printing array of integers in reversed order:
int[] array ={12,52,83,14,55};
Console.WriteLine("Reversed: ");
for (int i = array.Length-1; i >= 0; i--)
{
Console.Write(array[i] + " ");
}
 Print all elements of a string[] array using foreach:
string[] cities= { “Adama”, “Hawassa", “Debre Berhan", “Bahir Dar", “Mekelle"};
foreach (string city in cities)
{
Console.WriteLine(city);
}
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Multidimensional Arrays
 Using Array of Arrays, Matrices and Cubes
 Multidimensional arrays have more than one dimension (2, 3, …)
 The most important multidimensional arrays are the 2-dimensional
> Known as matrices or tables
 Declaring multidimensional arrays:
int[,] intMatrix;
float[,] floatMatrix;
string[,,] strCube;
 Creating a multidimensional array
 Use new keyword
 Must specify the size of each dimension
int[,] intMatrix = new int[3, 4];
float[,] floatMatrix = new float[8, 2];
string[,,] stringCube = new string[5, 5, 5];
 Creating and initializing with values multidimensional array:
int[,] matrix =
{
{1, 2, 3, 4}, // row 0 values
{5, 6, 7, 8}, // row 1 values
}; // The matrix size is 2 x 4 (2 rows, 4 cols)
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Multidimensional Arrays > Example
 Reading a matrix from the console
int rows = int.Parse(Console.ReadLine());
int cols= int.Parse(Console.ReadLine());
int[,] matrix = new int[rows, cols];
String inputNumber;
for (int row=0; row<rows; row++)
{
for (int col=0; col<cols; col++)
{
Console.Write("matrix[{0},{1}] = ", row, col);
inputNumber = Console.ReadLine();
matrix[row, col] = int.Parse(inputNumber);
}
}
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 Printing a matrix on the console:
for (int row=0; row<matrix.GetLength(0); row++)
{
for (int col=0; col<matrix.GetLength(1); col++)
{
Console.Write("{0} ", matrix[row, col]);
}
}
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 Finding a 2 x 2 platform in a matrix with a maximal sum of its
elements
int[,] matrix = {
{7, 1, 3, 3, 2, 1},
{1, 3, 9, 8, 5, 6},
{4, 6, 7, 9, 1, 0}
};
int bestSum = int.MinValue;
for (int row=0; row<matrix.GetLength(0)-1; row++)
for (int col=0; col<matrix.GetLength(1)-1; col++)
{
int sum = matrix[row, col] + matrix[row, col+1]
+ matrix[row+1, col] + matrix[row+1, col+1];
if (sum > bestSum)
bestSum = sum;
}
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Array Functions
 Array is a class; hence provides properties and methods to work with it
 Property
 Length – gets the number of elements in all the dimension of array
 Methods
 GetLength(dimension) - gets the number of elements in the specified dimension of an
array
 GetUpperBound(dimension) – Gets the index of the last elements in the specified
dimension of an array
 Copy(array1, array2, length) – Copies some or all of the values in one array to another
array.
 BinarSearch(array, value) – Searches a one-dimensional array that’s in ascending order and
returns the index for a value
 Sort(array) – Sorts the elements of a one dimensional array in to ascending order
 Clear(array, start, end) – clears elements of an array
 Reverse(array) - reverses the elements of an array
 The BinarySearch method only works on arrays whose elements are in ascending
order
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Array Function > Example
int [] number = new int [4] {1,2,3,4,5};
int len = numbers.GetLength(0);
int upperBound = numbers.GetUpperBound(0);
//
string[] names = {“Abebe”, “Kebede”, “Soliana”, “Fatuma”, “Makida”};
Array.Sort(names);
foreach(string name in names)
Console.WriteLine(name);
//
decimal[] sales = {15463.12m, 25241.3m, 45623.45m, 41543.23m,
40521.23m};
int index = Array.BinarySearch(names, “Soliana”);
decimal sale = sales[index];
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Copying Arrays
 Sometimes we may need to copy the values from one array to
another one
 If we do it the intuitive way we would copy not only the values but the
reference to the array
 Changing some of the values in one array will affect the other
> int[] copyArray=array;
 The way to avoid this is using Array.Copy()
> Array.Copy(sourceArray, copyArray);
 This way only the values will be copied but not the reference
 Reading assignment : How to work with Jagged arrays
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List<T>
 Lists are arrays that resize dynamically
When adding or removing elements
> use indexers ( like Array)
T is the type that the List will hold
> E.g.
 List<int> will hold integers
 List<object> will hold objects
 Basic Methods and Properties
Add(T element) – adds new element to the end
Remove(element) – removes the element
Count – returns the current size of the List
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List > Example
List<int> intList=new List<int>();
for( int i=0; i<5; i++)
{
intList.Add(i);
}
 Is the same as
int[] intArray=new int[5];
for( int i=0; i<5; i++)
{
intArray[i] = i;
}
 The main difference
 When using lists we don't have to know the exact number of elements
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Lists vs. Arrays
 Lets have an array with capacity of 5 elements
int[] intArray=new int[5];
 If we want to add a sixth element ( we have already added 5) we have
to do
int[] copyArray = intArray;
intArray = new int[6];
for (int i = 0; i < 5; i++)
{
intArray[i] = copyArray[i];
}
intArray[5]=newValue;
 With List we simply do
list.Add(newValue);
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ArrayList
 It represents ordered collection of an object that can be indexed
individually.
 It is basically an alternative to an array.
 However, unlike array you can add and remove items from a list
at a specified position using an index and the array resizes itself
automatically.
ArrayList arrayList = new ArrayList();
 It can contain a mixed content as object
 It also allows dynamic memory allocation, adding, searching and
sorting items in the list.
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ArrayList > Example
ArrayList al = new ArrayList();
Console.WriteLine("Adding some numbers:");
al.Add(45);
al.Add(78);
al.Add(33);
al.Add(56);
al.Add(12);
al.Add(23);
al.Add(9);
Console.WriteLine("Capacity: {0} ", al.Capacity);
Console.WriteLine("Count: {0}", al.Count);
Console.Write("Content: ");
foreach (int i in al)
{
Console.Write(i + " ");
}
Console.Write("Sorted Content: ");
al.Sort();
foreach (int i in al)
{
Console.Write(i + " ");
}
ArrayList arrrayList = new ArrayList();
arrrayList.Add(“One”);
arrrayList.Add(2);
arrrayList.Add(“Three”);
arrrayList.Add(4);
int number = 0;
foreach(object obj in araayList)
{
If(obj is int)
{
number += (int) obj;
}
}
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Enumeration - enum
 Enumeration is a set of related constants that define a value type
 Each constant is a member of the enumeration
 syntax
enum enumName [: type]
{
ConstantName1 [= value1],
ConstantName2 [=value2], …
}
 Example
enum Days { Sun, Mon, tue, Wed, thu, Fri, Sat };
int WeekdayStart = (int)Days.Mon;
int WeekdayEnd = (int)Days.Fri;
Console.WriteLine("Monday: {0}", WeekdayStart);
Console.WriteLine("Friday: {0}", WeekdayEnd);
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Dictionaries
 Dictionaries are used to associate a a particular key with a given
value
 In C#, defined by Hashtable class
 It uses a key to access the elements in the collection.
 A hash table is used when you need to access elements by using
key, and you can identify a useful key value.
 Each item in the hash table has a key/value pair.
 The key is used to access the items in the collection.
 Key must be unique
 Do not have any sense of order
4/13/2021 183

Hashtable > Example
Hashtable ht = new Hashtable();
ht.Add("001", “Abe Kebe");
ht.Add("002", “Alem Selam");
ht.Add("003", “Fatuma Ahmed");
ht.Add("004", “Soliana Abesselom");
ht.Add("005", “Tigist Abrham");
ht.Add("006", “Selam Ahmed");
ht.Add("007", “Makida Birhanu");
if (ht.ContainsValue(" Selam Ahmed "))
{
Console.WriteLine("This student name is already in the list");
}
else
{
ht.Add("008", " Selam Ahmed ");
}
// Get a collection of the keys.
ICollection key = ht.Keys;
foreach (string k in key)
{
Console.WriteLine(k + ": " + ht[k]);
}
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Stacks
 Stack also maintain a list of items like array and ArrayList, but
 Operates on a push-on and pop-off paradigm
 Stacks are LIFO – Last In First Out
Stack stack = new Stack();
stack.Push(“item”); // insert item on the top
object obj = stack.Pop(); // gets top item
object obp = stack.Peek() // looks at top
int size = stack.Count; //gets the number of items in the stack
4/13/2021 185

Queues
 Queues maintain a list of items like stack, but
 Operate with a principle of FIFO – First In First Out
Queue queue = new Queue();
queue.Enqueue(“item”); //insert an item to the last
object obj = queue.Dequeue(); // gets first item
int size = queue.Count; // gets number of items
4/13/2021 186

 Brian Bagnall, et al. C# for Java Programmers. USA. Syngress
Publishing, Inc.
4/13/2021 187

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