using System;

​

public interface IA

{

    void DoA();

}

public interface IB : IA

{

    void DoB();

}

public class B : IB, IA

{

    public void DoA() { Console.WriteLine("DoA in B."); }

    public void DoB() { Console.WriteLine("DoB in B."); }

}

​

public class C : B, IA

{

    void IA.DoA() { Console.WriteLine("DoA in C."); }

}

​

public class Program

{

    public static void Main()

    {

        var b = new B();

        b.DoA();

        b.DoB();

        var c = new C();

        c.DoA();

        (c as IA).DoA();

    }

}