using System;

public class Program

{

    public static void Main()

    {

        var b = new Boo();

        b.Call();

    }

}

​

public class Foo{

    protected virtual void Add(){

        Console.WriteLine("Hello");

        return;

    }

}

public class Boo : Foo{

    public void Call(){ Add(); return;}

    protected override void Add(){

        base.Add();

        Console.WriteLine("World");

        return;

    }

}