Mocking a method that takes an action which Sets a Threading.ManualResetEvent

MockActionCallback.cs

//code from NServicebus....
public static int WaitForReturn(this ICallback callback, TimeSpan timeout)
    {
      int returnValue = 0;
      ManualResetEvent handle = new ManualResetEvent(false);
      callback.Register((Action<int>) (i =>
      {
        returnValue = i;
        handle.Set();
      }));
      handle.WaitOne(timeout);
      return returnValue;
    }
    
    
//In order to mock this....

var messageCallback = new Mock<ICallback>();
messageCallback.Setup(m => m.Register(It.IsAny<Action<int>>()))
                                  //this has to be on a separate thread or else it will block forever.
    .Callback<Action<int>>(a => System.Threading.ThreadPool.QueueUserWorkItem(state =>
    {
        System.Threading.Thread.Sleep(5); //You have to give the original thread time to execute the handle.waitone. 
        a(123); 
    }));
//Set up the queue mock so it returns the ICallback mock.
messageDispatcher.Setup(md => md.Queue(It.IsAny<IMessage[]>()))
    .Returns(messageCallback.Object);