An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn’t support multiple inheritance, interfaces are used to implement multiple inheritance.

What is Shadowing?

Shadowing is a concept of polymorphism usage in Object Oriented Programming.
This is a concept related to over-riding functions at run-time or making a shadow of the object's methods in the inherited classes.
Let me explain you in terms of technology now. Though it is not much used in programming but it may be used sometimes and its usage is very restrictive and unique.

Implementation of override to override a method
Suppose you have a base class ClassA in which you have written a method MethodA. Now, if you want to inherit this class in a ClassB and use MethodA then the MethodA declared in ClassA will be used but if you override this method in your inherited class classB then it will be over-ridden and the implementation of classB will be used whenever we use it from classB and the implementation of classA will be used whenever we use it from a instance of classA.

using System;
using System.Diagnostics;
using System.IO;
public class ClassA {
public virtual void MethodA() {
Trace.WriteLine("BaseClass MethodA");
}
}

public class ClassB :ClassA{
public override void MethodA() {
Trace.WriteLine("SubClass MethodA overridden");
}
}

//implementation class for using the above defined classes
public class TopLevel {
static void Main(string[] args) {
TextWriter tw = Console.Out;
Trace.Listeners.Add(new TextWriterTraceListener(tw));

ClassA obj = new ClassB();
obj.MethodA(); // Output will be “Subclass MethodA overridden”
}
}

Shadowing instead of overriding
Upto now the code shown was for overriding a base class method with the child class method, But what will happen If you want to provide the base class behaviour instead, use the new directive, with or without virtual at the base class  Is this possible in C#? Yes, it is and that is the concept of shadowing in C# using a keyword "new' which is a modifier used instead of "override".

public class ClassA {
public virtual void MethodA() {
Trace.WriteLine("ClassA Method");
}
}

public class ClassB : ClassA {
public new void MethodA() {
Trace.WriteLine("SubClass ClassB Method");
}
}

public class TopLevel {
static void Main(string[] args) {
TextWriter tw = Console.Out;
Trace.Listeners.Add(new TextWriterTraceListener(tw));

ClassA obj = new ClassB();
obj.MethodA(); // Outputs “Class A Method"

ClassB obj1 = new ClassB();
obj.MethodA(); // Outputs “SubClass ClassB Method”
}
}

Over Loading and Overriding

In a class if two method is having the same name and different signature,its known as overloading in Object oriented concepts.

For eg  Take the case of a Shape Class. having  a method with a name DrawShape();

This method has two definitins with different parameters.

public void DrawShape(int x1, int y1,int x2,int y2)
{
 // draw a rectangle.
}

public void DrawShape(int x1,int y1)
{

// draw aline.
}

Overriding means, to give a specific definition by the derived class for a method implemented in the base class.

For eg.



Class Rectangle
{

   publc void DrawRectangle()
  {
    // this method will draw a rectangle.
  }

}


Class RoundRectangle : Rectanlge
{

  public void DrawRectangle()
  {

      //Here the DrawRectangle() method is overridden in the
     // derived class to draw a specific implementation to the
     //derived class, i.e to draw a rectangle with rounded corner.

  }
}

Over Loading and Overriding

In a class if two method is having the same name and different signature,its known as overloading in Object oriented concepts.

For eg  Take the case of a Shape Class. having  a method with a name DrawShape();

This method has two definitins with different parameters.

public void DrawShape(int x1, int y1,int x2,int y2)
{
 // draw a rectangle.
}

public void DrawShape(int x1,int y1)
{

// draw aline.
}

Overriding means, to give a specific definition by the derived class for a method implemented in the base class.

For eg.



Class Rectangle
{

   publc void DrawRectangle()
  {
    // this method will draw a rectangle.
  }

}


Class RoundRectangle : Rectanlge
{

  public void DrawRectangle()
  {

      //Here the DrawRectangle() method is overridden in the
     // derived class to draw a specific implementation to the
     //derived class, i.e to draw a rectangle with rounded corner.

  }
}

The OverLoading is nothing but the class which has a differ sign with the same method name. In overridiging is the method which is dervied from the base class which has been changed in sub class of it's functionality.

Dear Sujit kumar,

The following lines of your comments marked in red need to be corrected.