using System;

public class Program

{

    public static void Main()

    {

        var c = new C { Obj = new B() };

        var b = c.Obj as B;

        Console.WriteLine(b.Val);

    }

    public interface IMyInterface {}

    public class A : IMyInterface {}

    public class B : A {

        public string Val { get { return "Hello"; } }

    }

    public class C {

        public IMyInterface Obj {get;set;}

    }

}