Confused by the behavior of Dispatcher.BeginInvoke()

Question

Could someone shed some light on an issue I'm having?

I'm working on a wpf project. The scenario is as below:

I need to pop up a window(model window) on main UI thread and then close it. These works are started from another UI thread (to deter user from clicking on the main UI window.) then I close this window. The main code are displayed below. And it works.

As far as I know the close method would not get excuted before ShowDialog() returns (at least this is the case on UI thread, I mean code without dispatcher), does anyone have experience with multithread?

   Window window;
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        Thread thread = new Thread(() =>
           {


              //create a window and let user work from this thread

             //code is omitted.



               //create another window on main UI thread

              Application.Current.Dispatcher.BeginInvoke(new Action(() =>
                {
                    window = new Window();
                    window.ShowDialog();
                }));



               //do some work here

               Thread.Sleep(1000);

               Application.Current.Dispatcher.BeginInvoke(new Action(() =>
               {
                   //Thread.Sleep(1000);
                   window.Close();
               }));
           });

        thread.Start();
    }

Thank you for your time!

Answer 1

So if I understand your question correctly, you're saying that this code works exactly the way you want, but you're just trying to understand how (and why) it works?

Here's how it works. First, your thread runs this code:

Application.Current.Dispatcher.BeginInvoke(new Action(() =>
{
    window = new Window();
    window.ShowDialog();
}));

That queues your action on the main (UI) thread's dispatcher queue, and then returns immediately: your worker thread continues running.

When the Application first started up (typically via the compiler-generated code that initializes your App.xaml object, though you can also do it explicitly by calling Application.Run), it started its message loop, which goes something like this (pseudocode, very very simplified):

public class Application {
    public void Run() {
        while (!Exited && action = Dispatcher.DequeueAction())
            action();
    }
}

So at some point shortly after you queue the action, the UI thread will get around to pulling your action off the queue and running it, at which point your action creates a window and shows it modally.

The modal window now starts its own message loop, which goes something like this (again, very simplified):

public class Window {
    public bool? ShowDialog() {
        DisableOtherWindowsAndShow();
        while (!IsClosed && action = Dispatcher.DequeueAction())
            action();
        EnableOtherWindowsAndHide();
        return DialogResult;
    }
}

Later, your worker thread runs this code:

Application.Current.Dispatcher.BeginInvoke(new Action(() =>
{
    window.Close();
}));

Again, your action is queued to the UI thread's dispatcher queue, and then the BeginInvoke call returns immediately and your worker thread continues running.

So sooner or later, the UI thread's message loop will get around to dequeuing and executing your action, which tells the window to close. This has essentially the same effect as the user clicking the title bar's "X" button, which of course is perfectly OK to do even when you're inside a modal dialog. This causes ShowDialog's message loop to terminate (because the window is now closed), at which point the dialog is hidden and the other windows are re-enabled, ShowDialog returns, your original (ShowDialog) action is complete and so returns, and control falls back to the original message loop in Application.Run.

Note that there's one dispatcher queue per thread, not one per message loop. So your "close" action goes into the same queue that your "show dialog" action did. It's a different piece of code doing the message-loop polling now (the one inside ShowDialog instead of the one inside Application.Run), but the basics of the loop are the same.

Answer 2

BeginInvoke is a non-blocking method; it adds the action to the dispatcher queue, and doesn't wait for its completion. You should use Invoke instead, which calls the method synchronously on the dispatcher thread.