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));