using System;

using System.Linq;

using System.Collections.Generic;

​

public static class Extension { 

    public static void Test3(this A a) {

        Console.WriteLine("Test 3");

    }

}

​

public class A {

    public void Test1() {

        Console.WriteLine("Test 1");

    }

}

​

public class B : A {

    public void Test2() {

        Console.WriteLine("Test 2");

    }

}

​

public class Program

{

    public static void Main()

    {

        A a = new A();

        a.Test1();

        a.Test3();

        B b = new B();

        b.Test1();

        b.Test2();

        b.Test3();

    }

}