How to Resolve Deadlocks in STA Threads Using StaTaskScheduler and BlockingCollection?

StaTaskScheduler and STA thread message pumping

Question:

Deadlock occurs when using StaTaskScheduler and BlockingCollection on the STA thread due to lack of message pumping.

Solution:

Use MsgWaitForMultipleObjectsEx instead of CoWaitForMultipleHandles to implement a custom sync context with message pumping capabilities.

Step-by-step instructions:

1. Understand the issue: The STA TaskScheduler does not automatically pump messages on the STA thread.
2. Identified root cause: Blocking operation performed by BlockingCollection does not pump messages.
3. Create a custom sync context: Override SynchronizationContext.Wait to use MsgWaitForMultipleObjectsEx to implement message pumping and view/dispatch messages.
4. Install a custom synchronization context: Install a custom synchronization context on each STA thread created by the StaTaskScheduler.
5. Use a custom sync context: Use StaTaskScheduler with a custom sync context to schedule tasks to ensure message pumping.

Implementation instructions:

• Detect incoming messages using MWMO_INPUTAVAILABLE with the MsgWaitForMultipleObjectsEx flag.
• Manually pump messages by calling Wait and PeekMessage(PM_REMOVE) in the custom sync context's DispatchMessage method.
• Handle timeouts and potential race conditions gracefully.

Advantages:

• Resolves deadlock issues by ensuring messages are processed on the STA thread.
• Maintain the correct STA execution environment for legacy COM objects.
• Preserve thread affinity when using subsequent await continuations.

Sample code:

// 带有消息泵送的自定义同步上下文
class CustomSynchronizationContext : SynchronizationContext
{
    protected override void OnWait(bool unused)
    {
        const uint QS_MASK = 0x0001;
        const uint MWMO_INPUTAVAILABLE = 0x0004;

        var nativeResult = MsgWaitForMultipleObjectsEx(
            0, null, WAIT_INFINITE, QS_MASK, MWMO_INPUTAVAILABLE);

        var msg = new MSG();
        while (true)
        {
            if (nativeResult == WAIT_OBJECT_0)
                break;

            if (PeekMessage(out msg, IntPtr.Zero, 0, 0, PM_REMOVE))
            {
                TranslateMessage(ref msg);
                DispatchMessage(ref msg);
            }

            nativeResult = MsgWaitForMultipleObjectsEx(0, null, 0, QS_MASK, MWMO_INPUTAVAILABLE);
        }
    }
}

// 使用方法
using (var staThread = new ThreadWithAffinityContext(true, true))
{
    staThread.Run(async () =>
    {
        // 在 STA 线程上安装自定义同步上下文
        SynchronizationContext.SetSynchronizationContext(new CustomSynchronizationContext());

        // 使用消息泵送的 STA 线程代码...
    });
}

The above is the detailed content of How to Resolve Deadlocks in STA Threads Using StaTaskScheduler and BlockingCollection?. For more information, please follow other related articles on the PHP Chinese website!