Dispatcher to Thread relationships in WPF

Question

It is not entirely clear to me how many Dispatchers there are in an application and how they are related to, or referenced from Threads.

As I understand it, a WPF ap

Answer 1

WPF application by default has only one Dispatcher. The dispatcher is the only thread that will allow you to interact with UI elements. It abstracts implementations from you, so you only need to worry about being on the UI thread ie the Dispatcher.

If you are trying to directly interact with a visual (eg, set a text on a text box using txtBkx.Text = "new"), from a worker thread, then you will have to switch to a UI thread:

Application.Current.Dispatcher.Invoke(
    () => { txtBkx.Text = "new"; });

Alternatively you can use SynchronizationContext.Current (while on a UI thread) and use that to execute delegates on a UI thread from a different thread. As you should note that Dispatcher.CurrentDispatcher may not always be set.

Now you can in fact create different WPF windows in the same application and have an individual dispatcher for each window:

Thread thread = new Thread(() =>
{
    Window1 w = new Window1();
    w.Show();

    w.Closed += (sender2, e2) =>
    w.Dispatcher.InvokeShutdown();

    System.Windows.Threading.Dispatcher.Run();
});

thread.SetApartmentState(ApartmentState.STA);
thread.Start();  

As a side note remember in MVVM, you can update model from a non UI thread and raise property changed events from a non UI thread, as WPF will marshal PropertyChanged events for you. Raising CollectionChanged has to be on a UI thread though.