C# - Volatile Fields

Track/Stream: Moloko - Fun For Me

Fields in C# classes can be marked as volatile. I'm slightly unclear on this, but it looks like a niche task, really only needed for concurrent thread signalling issues. For example, if you have the following:

  class Foo

  {

    public int i = 0;

    public bool b = false;

    public void Go()

    {

      i = 42;

      b = true;

    }

    public int Wait()

    {

      while (!b)

        System.Threading.Thread.Sleep(1);

      return i;

    }

  }

If you call Go() from thread A, then call Wait() from thread B, it's possible to get 0 as your result. This is because .NET optimizes stuff behind the scenes, and assignment statements might not get executed in order behind the scenes. This is all covered when you're using a single thread (so if thread A checked these values, it'd be fine), but for multithreaded scenarios it isn't handled.

Now, if you were to put a volatile keyword on b (i.e. public volatile bool b; ), it would ensure that all assignments were flushed before b is assigned.

I'm sure there's an excellent reason behind this, but I'm equally sure I've pebcak'ed a bug or two due to this. Hmmmm, one more thing to watch out for.