A class is a blueprint that defines the data and behavior of a type through fields, methods, and events. It allows the creation of custom types by grouping these elements. Classes contain members like data fields and function members (methods, properties, etc.). Structs are similar to classes but are value types while classes are reference types. Methods define blocks of code that take parameters and return values. Properties provide a mechanism to read or write private fields through accessors. Constructors initialize objects when created and can call other constructors. Classes and types can be split across multiple files using partial classes.

2.  A class is a construct that enables you to
create your own custom types by grouping
together variables of other types, methods
and events.
 A class is like a blueprint. It defines the data
and behavior of a type.
 Syntax for creating a class
 [attributes] [modifiers] class identifier [:base-list]
{ class-body }
 Ex:
 public class Customer { //Fields, properties, methods and
events go here... }
Contd…

3.  The data and functions within a class are
known as the class’s members.
 Data Members
 Data members are those members that contain the data for
class – fields, constants and events.
 Data members can be static.
 Fields are any variables associated with the class.
 Events are class members that allow an object to notify a
caller whenever something of interest occurs.
 Function Members
 Function members are those members that provide
some functionality for manipulating the data in the
class.
 These include methods, properties, constructors,
finalizers, operators, and indexers.

4.  A struct type is a value type that can contain
constructors, constants, fields, methods, properties,
indexers, operators, events, and nested types.
 The declaration of a struct takes the following form:
 [attributes] [modifiers] struct identifier [:interfaces] {//body }
 Structs share most of the same syntax as classes,
although structs are more limited than classes:
 Within a struct declaration, fields cannot be initialized
unless they are declared as const or static.
 A struct cannot declare a default constructor (a
constructor without parameters) or a destructor.

5.  Structs are value types and classes are
reference types.
 Unlike classes, structs can be instantiated
without using a new operator.
 Structs can declare constructors that have
parameters.
 A struct cannot inherit from another struct or
class, and it cannot be the base of a class. All
structs inherit directly from
System.ValueType, which inherits from
System.Object.
 A struct can implement interfaces.
 A struct can be used as a nullable type and
can be assigned a null value.

6.  A method is a code block that contains a series of
statements.
 In C#, every executed instruction is performed in
the context of a method.
 Declaring Methods
◦ Syntax
[modifiers] return-type MethodName ([parameters])
{
//method body
}
Contd…

7.  Passing Parameters to Methods
◦ Parameters can be passed into methods by reference or by value.
◦ In C#, all parameters are passed by value unless you specifically
say otherwise.
 ref Parameters
◦ The ref keyword causes arguments to be passed by reference.
The effect is that any changes to the parameter in the method
will be reflected in that variable when control passes back to the
calling method.
 out Parameters
◦ The out keyword on a method parameter causes a method to
refer to the same variable that was passed into the method.
Any changes made to the parameter in the method will be
reflected in that variable when control passes back to the
calling method.
Contd…

8.  Passing by Reference vs. Passing by Value
 When a value type is passed to a method, a copy is
passed instead of the object itself. Therefore, changes
to the argument have no effect on the original copy in
the calling method. You can pass a value-type by
reference by using the ref keyword.
 Reference types are passed by reference. When an
object of a reference type is passed to a method, the
reference points to the original object, not a copy.
Changes made through this reference will therefore be
reflected in the calling method.
Contd…

9.  Named Arguments
 Named arguments allow you to pass in parameters in
any order.
 Optional Arguments
 Parameters can also be optional
 You must supply a default value for parameters that
are optional
 The optional parameter(s) must be the last ones
defined as well.

10.  C# supports method overloading – several
versions of the method that have different
signatures.
◦ To overload methods, you simply declare the methods with
the same name but different number or types of
parameters.

11.  A property is a member that provides a flexible
mechanism to read, write, or compute the value of
a private field.
 Properties can be used as if they are public data
members, but they are actually special methods
called accessors.
◦ Ex:
private int _age;
public int Age
{
get{return age;}
set{age=value;}
}
◦ We can have access modifiers for set and get accessors

12.  Whenever a class or struct is created, its constructor is called.
 A class or struct may have multiple constructors that take
different arguments.
 Constructors enable the programmer to set default values,
limit instantiation, and write code that is flexible and easy to
read.
 If you do not provide a constructor for your object, C# will
create one by default that instantiates the object and sets
member variables to the default values.
 Static classes and structs can also have constructors.
 We can have private constructors
Contd…

13.  Static Constructors
 A static constructor is used to initialize any static data, or to perform
a particular action that needs to be performed once only. It is called
automatically before the first instance is created or any static
members are referenced.
 A static constructor does not take access modifiers or have parameters.
 The user has no control on when the static constructor is executed in the
program.
 Calling Constructors from Other Constructors
 A constructor can use the base keyword to call the constructor of
a base class.
 Ex:
public Manager() : base()
{ //Add further instructions here. }
Contd…

14.  A constructor can invoke another constructor
in the same object by using the this keyword.

15.  The readonly keyword is a modifier that you can
use on fields.
 When a field declaration includes a readonly
modifier, assignments to the fields introduced by
the declaration can only occur as part of the
declaration or in a constructor in the same class.
 Ex:
class Age {
readonly int _year; Age(int year)
{ _year = year; }
void ChangeYear() { //_year = 1967; // Compile error if uncommented. }
}

16.  Anonymous types provide a convenient way to
encapsulate a set of read-only properties into a single
object without having to explicitly define a type first.
 The type name is generated by the compiler and is not
available at the source code level. The type of each
property is inferred by the compiler.
 You create anonymous types by using the new operator
together with an object initializer.
◦ Ex:
 var v = new { Amount = 108, Message = "Hello" };

17.  It is possible to split the definition of a class or a struct, an
interface or a method over two or more source files. Each
source file contains a section of the type or method
definition, and all parts are combined when the application is
compiled.
 To split a class definition, use the partial keyword modifier,
as shown here:
public partial class Employee
{
public void DoWork() { }
}
public partial class Employee
{
public void GoToLunch() { }
}

18.  A static class is basically the same as a non-static
class, but there is one difference: a static class
cannot be instantiated.
 The following list provides the main features of a static class:
 Contains only static members.
 Cannot be instantiated.
 Is sealed.
 Cannot contain Instance Constructors.

19.  As indicated earlier, all .NET classes are ultimately
derived from System.Object.
 System.Object Methods:
Equals(Object) Determines whether the specified object is equal to the
current Object.
Finalize() Allows an object to try to free resources and perform other
cleanup operations before it is reclaimed by garbage
collection.
GetHashCode() Serves as a hash function for a particular type
MemberwiseClone() Creates a shallow copy of the current Object.
ToString() Returns a string that represents the current object.

20.  Extension methods enable you to "add" methods to
existing types without creating a new derived type,
recompiling, or otherwise modifying the original type.
 Extension methods are defined as static methods but
are called by using instance method syntax.
 Their first parameter specifies which type the method
operates on, and the parameter is preceded by the
this modifier.
 Extension methods are only in scope when you
explicitly import the namespace into your source
code with a using directive.