WPF's multi-threaded UI is not thread safe

WPF supports multiple UI threads in its framework. You can create multiple UI thread windows or create multiple UI threads in a single window. But unfortunately, this is not really thread-safe.

There is a very low probability that WPF application will crash when you creating a multi-thread UI. In this post, I’ll tell how this happens.

The Issue

Necessary conditions:

1. Create multiple WPF UI threads
   • In fact, two are enough, one is the main UI thread with the App class we usually write; a background UI thread, for example, to display the UI thread that starts the splash screen.
   • If you use two threads, you need a lot of repetitive trials to reproduce; and by creating more threads you can greatly improve the probability of a single recurrence
2. These UI threads all display WPF windows
3. This issue will occur in both WPF on .NET Core 3 and WPF on .NET Framework 4.7.2.

phenomenon:

- An exception is thrown and the application crashes

For example, the following is one of the exceptions:

Exception thrown: 'System.NullReferenceException' in WindowsBase.dll
Object reference not set to an instance of an object.

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.IO.Packaging.PackagePart.CleanUpRequestedStreamsList()
   at System.IO.Packaging.PackagePart.GetStream(FileMode mode, FileAccess access)
   at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
   at Walterlv.Bugs.MultiThreadedUI.SplashWindow.InitializeComponent() in C:\Users\lvyi\Desktop\Walterlv.Bugs.MultiThreadedUI\Walterlv.Bugs.MultiThreadedUI\SplashWindow.xaml:line 1
   at Walterlv.Bugs.MultiThreadedUI.SplashWindow..ctor() in C:\Users\lvyi\Desktop\Walterlv.Bugs.MultiThreadedUI\Walterlv.Bugs.MultiThreadedUI\SplashWindow.xaml.cs:line 24
   at Walterlv.Bugs.MultiThreadedUI.Program.<>c__DisplayClass1_0.<RunSplashWindow>b__0() in C:\Users\lvyi\Desktop\Walterlv.Bugs.MultiThreadedUI\Walterlv.Bugs.MultiThreadedUI\Program.cs:line 33

The following image is an exception caught in WPF on .NET Core 3 that is shown in visual studio 2019:

How to Reproduce

1. Create a new WPF project (either .NET Core 3 or .NET Framework 4.7.2)
2. Keep the automatically generated App and MainWindow unchanged, we create a new window SplashWindow .
3. Create a new Program class containing the Main function and set Program as the startup object (instead of App ) in the project properties.

All other files remain the same as the default code generated by Visual Studio, and the code of Program.cs is as follows:

using System;
using System.Threading;
using System.Windows.Threading;

namespace Walterlv.Bugs.MultiThreadedUI
{
    public class Program
    {
        [STAThread]
        private static void Main(string[] args)
        {
            for (var i = 0; i < 50; i++)
            {
                RunSplashWindow(i);
            }

            var app = new App();
            app.InitializeComponent();
            app.Run();
        }

        private static void RunSplashWindow(int index)
        {
            var thread = new Thread(() =>
            {
                var window = new SplashWindow
                {
                    Title = $"SplashWindow {index.ToString().PadLeft(2, ' ')}",
                };
                window.Show();
                Dispatcher.Run();
            })
            {
                IsBackground = true,
            };
            thread.SetApartmentState(ApartmentState.STA);
            thread.Start();
        }
    }
}

Remarks: Even if you add this code just before the Splash Window creating, this exception still occurs.

SynchronizationContext.SetSynchronizationContext(
    new DispatcherSynchronizationContext(
        Dispatcher.CurrentDispatcher));

本文会经常更新，请阅读原文： https://walterlv.com/post/wpf-multi-thread-ui-is-not-thread-safe-en.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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