using System;

using System.Collections.Generic;

using System.Threading;

public class Program

{

    public static void Main()

    {

        a testa =new a();

        b testb = testa as b; //<=

    }

}

​

public class a {

  public void test(){

       Console.WriteLine("test");

  }

}

​

public class b {

  public void test(){

       Console.WriteLine("test");

  }

}

​