using System;

public class Program

{

    public static void Main()

    {

        ITest x = new ATest();

        Console.WriteLine(x.Do());

        x = new BTest();

        Console.WriteLine(x.Do());

        ATest y = new BTest();

        Console.WriteLine(y.Do());

        BTest z = new BTest();

        Console.WriteLine(z.Do());

        z = (BTest)y;

        Console.WriteLine(z.Do());

​

    }

}

​

interface ITest {

    string Do();

}

​

class ATest : ITest {

    public string Do() { return "A";}

}

​

​

class BTest : ATest {

    public new string Do() { return "B";}

}