using System;

​

public class A  

{  

    public virtual void Show(){

    Console.WriteLine("Calling A"); 

    }

}  

​

public class B:A

{

public override void Show()

{

    Console.WriteLine("CAlling B"); 

}

}

​

public class Program

{

    public static void Main()

    {

        B b=new B();

        b.Show(); // return Calling B

        A a=new B();

        a.Show(); // return calling B

        A aa=new A();

        aa.Show(); // return calling A

        //B aac=new A();

        //aac.Show(); //Error Cannot implicitly convert type 'A' to 'B'. An explicit conversion exists (are you missing a cast?)

    }

}