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WPF Main thread gets a deadlock when stylus input thread is waiting for the window to close #78

lindexi opened this Issue Dec 5, 2018 · 0 comments


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lindexi commented Dec 5, 2018

  • .NET Core Version: 3.0 Preview1
  • Windows version: (Windows 10 all versions / 7 SP1)
  • Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

Problem description:

We found two way that can make the main thread locked. And we can not write any code to solve it and it can only be circumvented.

The easiest way to reproduce this issue is to wait for the window in the main thread to close in the stylus input thread.

Actual behavior:

The first way

The first way is to write a custom class implementing StylusPlugIn and wait for a window to close in the OnStylusUp method.

Let's create a new empty window named FooWindow.

public class FooWindow : Window


Then we create a FooStylusPlugIn class to implement the StylusPlugIn with overriding the OnStylusUp method. We add some code to wait for the window to close by calling Invoke which will wait by pumping a new message loop.

public class FooStylusPlugIn : StylusPlugIn
    public FooStylusPlugIn(FooWindow fooWindow)
    	FooWindow = fooWindow;

    public FooWindow FooWindow { get; }

    /// <inheritdoc />
    protected override void OnStylusUp(RawStylusInput rawStylusInput)
        FooWindow.Dispatcher.Invoke(() => FooWindow.Close());

To combine both the critical codes above, we write some codes in the MainWindow. The FooWindow is instanced in the constructor and the StylusPlugIn is plugged in it. We also make a button in the XAML that can let us know whether the main thread is still running or not.

public partial class MainWindow : Window
    public MainWindow()
        _fooWindow = new FooWindow();
        StylusPlugIns.Add(new FooStylusPlugIn(_fooWindow));

    private void Button_OnClick(object sender, RoutedEventArgs e)

    private FooWindow _fooWindow;

Run the project, touch the main window, and you'll find that the main window never responds to your interaction. Try to click the button to view the responding and you'll soon verify what I'm talking.

The reason is that the OnStylusUp in FooStylusPlugIn is running in the stylus input thread which is also running the inside loop of the ThreadProc method. It needs to go back to the outside loop to remove the PenContext when a window is closed. The stylus input thread is waiting for the main thread to close a window and the main thread is also waiting for the stylus input thread remove PenContext. Thus, the deadlock occurred.

The demo in github

The second way

If a touch happens exactly during a window closing, the main thread will enter a lock.

The difference between the first method and the second method is that the first one will lock both the main thread and the stylus input thread but the second one will only lock the main thread.

From the theory, we know that the PenContext should be removed correctly in the outside loop. But in the second way, the stylus input thread is firing the touch event exactly when we run the code to remove the PenContext in the stylus input thread. As you can see we need to run the code to remove PenContext in the outside loop but at this moment the code is firing the touch event in the second loop.

The firing of the touch event means the _pimcResetHandle is released. Although the main thread has also released the lock the code cannot run to the outside loop to remove the PenContext and the main thread can no longer wait for the moment when the PenContext removal is finished.

void ThreadProc()
    while (!__disposed)
      	// The outside loop
    	// To remove or add the PenContext
    	// The main thread is waiting for its finishing.

    	while (true)
    		// The inside loop
    		// Tt will be blocked by the PENIMC
    		if(!Penimc.UnsafeNativeMethods.GetPenEvent(/*wait the lock*/))
    			// If the `_pimcResetHandle` is released, this if branch will enter so the inside loop will end with the `break` and the code runs back to the outside loop.

    		FireEvent(/*fire the touch events*/); // the code is running in this line
    		// and the `_pimcResetHandle` is released.
    		// the main thread release the `_pimcResetHandle` but the code can not go to RemovePenContext for it will no longer break. 

The main thread has released the lock but the stylus input thread doesn't need to wait for the lock. The stylus input thread cannot go back to the outside loop to remove the PenContext and main thread can no longer wait for the moment when the PenContext removal is finished.

Expected behavior:

Minimal repro:


@vatsan-madhavan vatsan-madhavan added the bug label Dec 5, 2018

@vatsan-madhavan vatsan-madhavan added this to the 3.0 milestone Dec 5, 2018

@rladuca rladuca modified the milestones: 3.0, Future Dec 5, 2018

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