lol.cs

using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {
            B b = new C();
            b.hi();

            Console.ReadLine();
        }   
    }

    class A
    {
        virtual public void hi()
        { Console.WriteLine("hiA"); }
    }

    abstract class B : A
    {
        override public abstract void hi();
    }

    class C : B
    {
        override public void hi()
        { Console.WriteLine("hiC"); }
    }
}