explain how it is possible overriding in a linear hierarchy

using System;

class class1

{

   public virtual void show()

   {

      Console.WriteLine("Hello from class1");

   }

}

 

class class2 : class1

{

   public override void show ()

   {

      Console.WriteLine("Hello from class2");

   }

}

class class3 : class2

{

   public override void show()

   {

      Console.WriteLine("Hello from class3");

   }

}

class class4 : class3

{

   public override void show()

   {

      Console.WriteLine("Hello from class4");

   }

}

class Test

{

   static void Main()

   {

      class1 a = new class1();

      class1 b = new class2();

      class2 c = new class3();

      class3 d = new class4();

      a.show(); // a is really a class1

      b.show(); // b is actually a class2

      c.show(); // c is actually a class3

      d.show(); // d is actually a class4

      Console.ReadKey();

   }

}

The output is:


The reason for this state of affairs is that when you have a virtual or override method, the CLR always looks at the run time type of the variable, not its compile time type, in deciding which version of the  method is to be called.

The run time type for this purpose is the actual type of object to which the variable refers.

So, in the case of these lines:

class3 d = new class4();
d.show();

whilst the compile type of 'd' is class3, its run time type is class4 and so it's class4's show method which gets called.