Can/Does WPF have multiple GUI threads? Or does it always only have one GUI thread (even if I have multiple windows/dialogs)?
I'm asking because I have events coming from other threads and I'd like to handle them in the GUI thread (because I need to modify the controls of my main window accordings to the events).
Btw: I know I need to use a Dispatcher object for this purpose. So, I could rephrase my question and ask: Is there always only one Dispatcher object for all GUI elements in WPF?
Based on the link in the first answer I did some verification on my own. I'd like to share the results here. First of all:
There can be multiple GUI threads (and therefor multiple Dispatcher instances).
However:
Simply creating a new window (modal or not) does not create a new GUI thread. One needs to create the thread explicitly (by creating a new instance of Thread).
Note: Instead of using separate threads, modal dialogs are likely being realized by using Dispatcher.PushFrame() which blocks the caller of this method while still allowing events to be dispatched.
I've created a simple WPF class (again, based on the link from the first answer) to verify all this stuff. I share it here so you can play around with it a little bit.
MainWindow.xaml:
MainWindow.xaml.cs:
using System; using System.Threading; using System.Windows; using System.Windows.Media; namespace WindowThreadingTest { /// /// Interaction logic for MainWindow.xaml /// public partial class MainWindow : Window { private static uint s_windowNumber = 0; private readonly MainWindow m_prevWindow; public MainWindow() : this(null) { } public MainWindow(MainWindow prevWindow) { InitializeComponent(); this.m_prevWindow = prevWindow; this.Title = String.Format("Window {0}", ++s_windowNumber); Thread thread = Thread.CurrentThread; this.m_threadId.Text = thread.ManagedThreadId.ToString(); this.m_threadTA.Text = thread.GetApartmentState().ToString(); } private void OnCreateNewWindow(object sender, RoutedEventArgs e) { CreateNewWindow(true, false, true); } private void CreateNewWindow(bool newThread, bool modal, bool showInTaskbar) { MainWindow mw = this; if (newThread) { Thread thread = new Thread(() => { MainWindow w = new MainWindow(this); w.ShowInTaskbar = showInTaskbar; if (modal) { // ShowDialog automatically starts the event queue for the new windows in the new thread. The window isn't // modal though. w.ShowDialog(); } else { w.Show(); w.Closed += (sender2, e2) => w.Dispatcher.InvokeShutdown(); System.Windows.Threading.Dispatcher.Run(); } }); thread.SetApartmentState(ApartmentState.STA); thread.Start(); } else { MainWindow w = new MainWindow(this); w.ShowInTaskbar = showInTaskbar; if (modal) { // Even modal dialogs run in the same thread. w.ShowDialog(); } else { w.Show(); } } } private void OnAccessTest(object sender, RoutedEventArgs e) { if (m_prevWindow == null) { return; } this.Background = Brushes.Lavender; try { m_prevWindow.Background = Brushes.LightBlue; } catch (InvalidOperationException excpt) { MessageBox.Show("Exception: " + excpt.Message, "Invalid Operation"); } m_prevWindow.Dispatcher.Invoke((Action)(() => m_prevWindow.Background = Brushes.Green)); this.Dispatcher.Invoke((Action)(() => { try { m_prevWindow.Background = Brushes.Red; } catch (InvalidOperationException excpt) { MessageBox.Show("Exception: " + excpt.Message, "Invalid Dispatcher Operation"); } })); } } }