example.cs

using System;

namespace extensions
{
    class Foo
    {
        private int myInt = 3;
        virtual public int MyInt
        {
            get { return myInt; }
        }
    }

    class ChildFoo : Foo
    {
        private int testInt = 0;

        public new int MyInt 
        {
            get { return testInt; }
            set { testInt = value; }
        }
    }

    class Program
    {
        static void showFoo(Foo f)
        {
            Console.WriteLine("i = " + f.MyInt);
        }

        static void Main(string[] args)
        {
            var f = new ChildFoo();
            showFoo(f);
            f.MyInt = 99;
            showFoo(f);
            Console.ReadLine();
        }
    }
}