using System;

using System.Linq;

public interface IA

{

​

    void A1();

    void A2();

}

public class B : IA

{

    public void A1()

    {

        Console.WriteLine("A1");

    }

    public void A2()

    {

        Console.WriteLine("B1");

    }

}

public abstract class A

{

    public int a;

    public abstract void B1();

    public abstract int B2();

}

​

public class Program

{

​

    public static void Main()

    {

        IA ia ;

        ia= new B();

        ia.A1();

        ia.A2();

    }

}