How to execute code in the GUI Thread?

To execute the extendedNotifyIcon_OnHideWindow method on the GUI thread use the Dispatcher as you did to show it.

Thread threadToClosePopup = new Thread(new ThreadStart((Action)delegate() { 
  Thread.Sleep(1000); 
  txtLog.Dispatcher.Invoke(
    DispatcherPriority.Normal,
    (Action)() => extendedNotifyIcon_OnHideWindow());
}));

Use Control.Invoke

   txtLog.Dispatcher.Invoke(DispatcherPriority.Normal, (Action)delegate() {
        this.txtLog.Text = dataToDisplay;
        extendedNotifyIcon_OnShowWindow();
        Thread threadToClosePopup = new Thread(new ThreadStart((Action)delegate() { 
            Thread.Sleep(1000); 

            extendedNotifyIcon.Invoke(extendedNotifyIcon_OnHideWindow);
   }));
        threadToClosePopup.Start();
    });

This will give you the Window dispatcher:

Dispatcher.CurrentDispatcher

As long as you get it on the windows thread.

This works well for WPF with MVVM.

Application.Current.Dispatcher.Invoke(
    () =>
    {
         // Code to run on the GUI thread.
    });

This will not work consistently (it will fail if we are inside a handler for Reactive Extensions):

Dispatcher.CurrentDispatcher.Invoke(
    () =>
    {
         // Fails if we are inside a handler for Reactive Extensions!
    });

Extra for Experts: The Reason?

By design, any thread can have a dispatcher thread paired with it, see MSDN: Dispatcher Class.

If we reference Dispatcher.CurrentDispatcher from any thread, it will actually create a new dispatcher thread, which is separate to the official WPF UI dispatcher thread. When we try to execute code on this newly created dispatcher thread, WPF will throw as it's not the official UI dispatcher thread.

The solution is to always use Application.Current.Dispatcher.Invoke(), or Application.Current.Dispatcher.BeginInvoke(). See What's the difference between Invoke() and BeginInvoke().

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Update 2020-05-02: It is possible to run a WPF application with multiple WPF UI dispatcher threads. I work with a huge WPF app that is doing this. It is tricky to get it to work, but once it works - it's amazing and the entire app runs an order of magnitude faster as there are multiple UI dispatcher threads. Happy to answer questions on this.

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Tested on:

• WPF
• .NET 4.5
• .NET 4.6
• .NET 4.61