using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var a = new B();

        a.M1("Jenish", null);

        a.M1("Jenish", "Rabadiya");

    }

}

​

public abstract class A

{

    public abstract void M1(string s1);

    public void M1(string s1, string s2)

    {

        if(s2 == null)

        {

            M1(s1);

        }

        else

        {

            Console.WriteLine("s1 from A : {0}", s1);

        }

    }

}

​

public class B : A

{

    public override void M1(string s1)

    {

        Console.WriteLine("s1 from B : {0}", s1);

    }

}

​

​