000IBB000.cs

using System;

namespace k
{
    public class B
    {
        public void Test()
        {
            Console.WriteLine("B");
        }
    }

    public class I : B
    {
        public void Test()
        {
            Console.WriteLine("I");
        }
    }

    class Program
    {
        static void Main()
        {
            I i = new I();
            i.Test();
            ((B) i).Test();
            B b = i;
            b.Test();
        }
    }
}