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:

<code class="language-csharp">// 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...
    });
}</code>

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