Here's a very simple example of an abstract class in C#.

Notice that abstract methods are virtual and therefore exhibit polymorphic behaviour. However, unlike a normal 'virtual' method, an abstract method cannot have a body.

Hi Fire,

Abstract class: - It is an incomplete class whose object cannot be created. It contains the skeleton of the method, which is implemented by derived class. The abstract class is prefixed by the keyword MustInherit. The methods with in the class are prefixed by MustOverride keyword. The derived class procedure, which implements the abstract class, is prefixed by Overrides keyword.

Program

Public Class Form1
 
  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
  Dim a As New raj1
  a.ravi("Rahul")
  End Sub
 
  Public MustInherit Class raj
  Public MustOverride Sub ravi(ByVal s As String)
  End Class
 
  Public Class raj1
  Inherits raj
  Public Overrides Sub ravi(ByVal s As String)
  MsgBox("Hello:" & s)
  End Sub
  End Class
 
End Class
 
Interface: - It is a specification of class members not the implementation of members.
It is the substitute of multiple inheritance, more than one interface can be implemented by a single class. The members with in the interface are not defined. It is public by default. Variables cannot be declared within the interface. Both sub procedure and function procedure can be stored in the interface.

Program

Public Class Form2
 
  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
  Dim a As New raj1
  a.sagar(25)
  a.amit("Abhijeet")
  MsgBox(a.rahul(10, 20))
 
  End Sub
  Public Interface raj
 
  Sub sagar(ByVal x As Integer)
  Sub amit(ByVal s As String)
  Function rahul(ByVal x As Integer, ByVal y As Integer)
 
  End Interface
 
  Public Class raj1
  Implements raj
 
  Public Sub amit(ByVal s As String) Implements raj.amit
 
  MsgBox("Hello:" & s)
 
  End Sub
 
  Public Function rahul(ByVal x As Integer, ByVal y As Integer) As Object Implements raj.rahul
 
  Dim z As Integer
  z = x + y
  Return z
 
  End Function
 
  Public Sub sagar(ByVal x As Integer) Implements raj.sagar
 
  MsgBox(x)
 
  End Sub
 
  End Class
 
End Class

For Csharp code Please refer the below links

http://www.codeproject.com/Articles/6118/All-about-abstract-classes

http://www.c-sharpcorner.com/uploadfile/annathurai/abstract-class-in-C-Sharp/

Thanks