using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var thing = new Thing() { Data = "Hello World!" };

        SomeAbstractOperation(thing);

        var shadowThing = new ShadowThing() { Data = "How are you?" };

        SomeAbstractOperation(shadowThing);

    }

    private static void SomeAbstractOperation<T>(T thing)

        where T : Thing

    {

        if (thing.Data == null)

            Console.WriteLine("OH NOES");

        else

            Console.WriteLine(thing.Data);

    }

​

    public class Thing

    {

        public object Data { get; set; }

    }

    public class ShadowThing : Thing

    {

        public new string Data { get; set; }

    }

}