Running Window in another Thread results in a crash

[janwiesemann]

After updating to 2.4.4 form 2.4.3 following code stopped working:

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        ShutdownMode = ShutdownMode.OnExplicitShutdown;
        StartupUri = null;
        
        base.OnStartup(e);

        //this works
        OpenWindow(false);

        //this does not work
        Thread t = new Thread(() =>
        {
            System.Windows.Threading.Dispatcher disp = System.Windows.Threading.Dispatcher.CurrentDispatcher; //Create new Dispatcher

            try
            {
                OpenWindow(true);
            }
            catch (Exception ex)
            {
                System.Diagnostics.Debugger.Break();
            }
            finally
            {
                disp.InvokeShutdown();
            }
        });
        t.SetApartmentState(ApartmentState.STA);
        t.IsBackground = true;
        t.Start();
    }

    private void OpenWindow(bool dialog)
    {
        MetroWindow wind = new MetroWindow();
        wind.Width = 100;
        wind.Height = 100;

        if (dialog)
            wind.ShowDialog();
        else
            wind.Show();
    }
}

Running this results in a InvalidOperationException at

   bei System.Windows.Threading.Dispatcher.VerifyAccess()
   bei System.Windows.Application.add_SessionEnding(SessionEndingCancelEventHandler value)
   bei MahApps.Metro.Behaviors.WindowsSettingBehavior.<AssociatedObject_SourceInitialized>b__2_0() in /_/src/MahApps.Metro/Behaviors/WindowsSettingBehavior.cs:Zeile 55.
   bei System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   bei System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   bei System.Windows.Threading.DispatcherOperation.InvokeImpl()
   bei System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   bei MS.Internal.CulturePreservingExecutionContext.CallbackWrapper(Object obj)
   bei System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   bei MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
   bei System.Windows.Threading.DispatcherOperation.Invoke()
   bei System.Windows.Threading.Dispatcher.ProcessQueue()
   bei System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   bei MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   bei MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   bei System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   bei System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   bei System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   bei MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   bei MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   bei System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   bei System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   bei System.Windows.Window.ShowHelper(Object booleanBox)
   bei System.Windows.Window.Show()
   bei System.Windows.Window.ShowDialog()
   bei MahError.App.OpenWindow(Boolean dialog) in C:\Users\jan.wiesemann\Desktop\MahError\MahError\App.xaml.cs:Zeile 50.
   bei MahError.App.<OnStartup>b__0_0() in C:\Users\jan.wiesemann\Desktop\MahError\MahError\App.xaml.cs:Zeile 27.

Due to some complex layouts I'm forced to run it in a different Thread. Otherwise all Windows are blocked white WPF initialises the other Window.

[chei90]

Holy Shit! That's exactly my problem.

A minimum crashing example from me would be something like this:

var thread = new Thread(() =>
{
	host = new MetroWindow()
	{
		Width = 200,
		Height = 100
	};
	host.Show();
	Dispatcher.Run();

});

thread.SetApartmentState(ApartmentState.STA);
thread.IsBackground = true;
thread.Start();

Edit2: Yes, a Downgrade to 2.4.3 circumvents the aforementioned crash.

[NimbusHex]

It's possible something did change from 2.4.3 to 2.4.4, but this is pretty standard WPF behavior. WPF requires you to be on the dispatcher thread when doing any UI actions. What you guys are doing with Dispatcher is a bit confusing to me, @Jan18101997's dispatcher code wouldn't be doing anything, as the dispatcher isn't being used in any way. @chei-de's Dispatcher.Run() code really does nothing in the context you're using it, host.Show would be causing the exception before you even hit that line of code anyway.

My guess would be that there used to be some sort of "ensure on dispatch thread" code in MetroWindow.Show() that was either taken out or broken in the 2.4.4 release. I'm pretty certain if you (@chei-de) wrap host.Show() in Dispatcher.BeginInvoke, the crash won't occur.

[janwiesemann]

It's possible something did change from 2.4.3 to 2.4.4, but this is pretty standard WPF behavior. WPF requires you to be on the dispatcher thread when doing any UI actions. What you guys are doing with Dispatcher is a bit confusing to me, @Jan18101997's dispatcher code wouldn't be doing anything, as the dispatcher isn't being used in any way. @chei-de's Dispatcher.Run() code really does nothing in the context you're using it, host.Show would be causing the exception before you even hit that line of code anyway.

My guess would be that there used to be some sort of "ensure on dispatch thread" code in MetroWindow.Show() that was either taken out or broken in the 2.4.4 release. I'm pretty certain if you (@chei-de) wrap host.Show() in Dispatcher.BeginInvoke, the crash won't occur.

Dispatcher.run starts a new dispatcher inside a new thread.

a change in 2.4.4 broke this. As far as I understand the code at

bei System.Windows.Application.add_SessionEnding(SessionEndingCancelEventHandler value)
   bei MahApps.Metro.Behaviors.WindowsSettingBehavior.<AssociatedObject_SourceInitialized>b__2_0() in /_/src/MahApps.Metro/Behaviors/WindowsSettingBehavior.cs:Zeile 55.

Access application.current.... but in this chase the main application instance is running in a different thread and will result in a crash.

[NimbusHex]

Dispatcher.run starts a new dispatcher inside a new thread.

Wow, I'm embarrassed to say I never knew this was possible in WPF, I thought you could only have a single Dispatcher per application. Thank you for the info!

[erotavlas]

I'm getting this exact error. My app used to work before however after upgrading to 2.4.4 this happened (except I upgraded from 1.6.0). I guess I'll downgrade to 2.4.3 as mentioned above.

[janwiesemann]

I'm getting this exact error. My app used to work before however after upgrading to 2.4.4 this happened (except I upgraded from 1.6.0). I guess I'll downgrade to 2.4.3 as mentioned above.

I've created a pull request for this issue but it was not accepted yet.

If you are upgrading from 1.x make sure to follow there upgrade guide. It's possible, that you have to change some resources as well.

Downgrading to 2.4.3 fixed the issue for me.