WPF shows that some windows in multithreading will be locked in the PenThreadWorker constructor when the application starts

Problem description: WPF will initialize pen thread when the window initializing. But I find the window may be locked in PenThreadWorker constructor. The UnsafeNativeMethods.CreateResetEvent will lock the thread.

Actual behavior: I use multithreading to create some windows by this code and maybe the thread be locked in PenThreadWorker constructor. The Demo code

    public partial class App : Application
    {
        /// <inheritdoc />
        protected override void OnStartup(StartupEventArgs e)
        {
            var thread = new Thread(() =>
            {
                var mainWindow = new MainWindow();
                mainWindow.Show();
                Dispatcher.Run();
            });
            thread.SetApartmentState(ApartmentState.STA);
            thread.Start();
            base.OnStartup(e);
        }
    }

The step

Run the code

Use the VisualStudio to suspend the application Maybe you can find the application only create a Stylus Input thread and show only a main window and the other main window and the thread be locked in PenThreadWorker constructor and the window can not show Why I need multithreading to show the windows? I need the splash window to show the welcome page and then I should show the main window in other thread to do the bussiness code and I should close the splash window when the main window showed. But I found some users could not show the main window and the main window be lock in PenThreadWorker constructor. I use dnspy to suspend the application and find the code run in PenThreadWorker constructor and the UnsafeNativeMethods.CreateResetEvent will lock the thread.

 PenThreadWorker..ctor()  
 PenThread..ctor()  
 PenThreadPool.GetPenThreadForPenContextHelper(PenContext penContext)  
 PenThreadPool.GetPenThreadForPenContext(PenContext penContext)  
 WispTabletDeviceCollection.UpdateTabletsImpl()  
 WispTabletDeviceCollection.UpdateTablets()  
 WispTabletDeviceCollection..ctor()  
 WispLogic.get_WispTabletDevices()  
 WispLogic.RegisterHwndForInput(InputManager inputManager, PresentationSource inputSource)  
 HwndStylusInputProvider..ctor(HwndSource source)  
 HwndSource.Initialize(HwndSourceParameters parameters)  
 HwndSource..ctor(HwndSourceParameters parameters)  
 PresentationFramework.dll!System.Windows.Window.CreateSourceWindow(bool duringShow)  
 PresentationFramework.dll!System.Windows.Window.CreateSourceWindowDuringShow()  
 PresentationFramework.dll!System.Windows.Window.ShowHelper(object booleanBox)  

Expected behavior: The UnsafeNativeMethods.CreateResetEvent do not lock the thread

Minimal repro:

Create a empty WPF application and then change the app.xaml.cs code

    public partial class App : Application
    {
        /// <inheritdoc />
        protected override void OnStartup(StartupEventArgs e)
        {
            var thread = new Thread(() =>
            {
                var mainWindow = new MainWindow();
                mainWindow.Show();
                Dispatcher.Run();
            });
            thread.SetApartmentState(ApartmentState.STA);
            thread.Start();
            base.OnStartup(e);
        }
    }

Run the code and maybe you can find one of the main window can not show and it stop in UnsafeNativeMethods.CreateResetEvent and I wait for a hour but it can not run.

本文会经常更新，请阅读原文： https://blog.lindexi.com/post/WPF-shows-that-some-windows-in-multithreading-will-be-locked-in-the-PenThreadWorker-constructor-when-the-application-starts.html ，以避免陈旧错误知识的误导，同时有更好的阅读体验。

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