Volatile.cs

using System;
using System.Threading;

class Program
{
    class Flag
    {
        public bool done;
    }

    static void Main(string[] args)
    {
        var f = new Flag { done = false };
        new Thread(() => Runner(f)).Start();

        Thread.Sleep(1000);
        f.done = true;

        Console.ReadKey();
    }

    static void Runner(Flag f)
    {
        Console.WriteLine("Start");

        while (!f.done) ;

        Console.WriteLine("Done");
    }
}