Question

Taking CPU caching and any possible compiler optimizations into account:

1. Is it possible for “False” to be written to the console?
2. Is option 1 setting up a memory fence that guarantees the freshness of the object reference and the field?

3. Is option 2 less “safe” than option 1 in any way?

   class Program
   {
       class Test
       {
           internal bool value = false;
       }
   
       static void Main(string[] args)
       {
           Test test = new Test();
   
           //Option 1
           ThreadPool.QueueUserWorkItem((s) => 
           {
               Thread.Sleep(1000);
               Console.WriteLine(((Test)s).value); 
           }, test);
   
           //Option 2
           ThreadPool.QueueUserWorkItem((s) =>
           {
               Thread.Sleep(1000);
               Console.WriteLine(test.value);
           });
   
           test.value = true;
           Console.ReadLine();
       }
   }
   

Answer 1

1. Yes, of course. There is no synchronization to prevent either option #1 or option #2’s worker threads from printing out the false value before the main thread sets it to true. While the contrived example uses a Thread.Sleep to prevent it from ever realistically occurring, there is no real synchronization here.

2. No, option 1 is simply sending a reference to s to the thread pool for forwarding to the delegate. Any memory fencing that happens should be considered an implementation detail, and as far as I am aware none occurs. There is certainly nothing (other than the aforementioned Thread.Sleep()) preventing option #1’s worker thread from printing “false” before the main thread sets it to true.

3. Option 2 is less safe in that you are closing over a variable which may be modified in a less contrived example. See Eric Lippert’s blog post on the subject of closing over loop variables– the same advice applies to option #2.