Why does Dispatcher.Invoke not execute the delegate argument in this example ?

Question

    [Test]
    public void A()
    {
        var d = Dispatcher.CurrentDispatcher;

        Action action = () => Console.WriteLine("Dispatcher invoked me!");

        var worker = new BackgroundWorker();
        worker.DoWork += SomeWork;

        //worker.RunWorkerAsync( (Action) delegate { Console.WriteLine("This works!"); } );
        worker.RunWorkerAsync((Action) delegate { d.Invoke(action); } );

        System.Threading.Thread.Sleep(2500);
    }

    private void SomeWork(object sender, DoWorkEventArgs e)
    {
        (e.Argument as Action)();
    }

This block of code doesn't throw an exception. At the same time, Dispatcher.Invoke does nothing. I found that odd.

I extracted a helper method into a base ViewModel. Worker threads used this method DoOnUIThread() to avoid the thread affinity issue. However in my unit-tests, I find that attempting to test the view model objects results in failures due to the above issue.

I could move this whole behavior out into a pluggable dependency that I could substitute in my tests. e.g. ViewModelBase depends on UIThreadExecutor.Execute(Action) and I use a fake that just calls the action in my tests. However I'm curious as to why Dispatcher behaves the way it does..

Answer 1

It looks like your just passing it as a parameter to the BackgroundWorker. Try adding this in your SomeWork function:

  public void SomeWork(object sender, DoWorkEventArgs e)
  {
     ((MulticastDelegate)e.Argument).DynamicInvoke();
  }

Instead of a straight Sleep you could try this:

  while (worker.IsBusy)
  {
     Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.Background,new EmptyDelegate(delegate{}));
     //Application.DoEvents();
     System.Threading.Thread.Sleep(10);
  }

Answer 2

Dispatcher can only perform its Begin/Invoke() duty when the main thread goes idle and re-enters the dispatch loop. At that point the main thread is quiescent and it can safely execute the dispatched requests. As well as any notifications sent to it by Windows.

It isn't idle in your case, it is stuck inside of Sleep(2500).