StaTaskScheduler and STA thread message pumping

My understanding of your problem: you are using StaTaskScheduler only to organize the classic COM STA apartment for your legacy COM objects. You’re not running a WinForms or WPF core message loop on the STA thread of StaTaskScheduler. That is, you’re not using anything like Application.Run, Application.DoEvents or Dispatcher.PushFrame inside that thread. Correct me if this is a wrong assumption.

By itself, StaTaskScheduler doesn’t install any synchronization context on the STA threads it creates. Thus, you’re relying upon the CLR to pump messages for you. I’ve only found an implicit confirmation that the CLR pumps on STA threads, in Apartments and Pumping in the CLR by Chris Brumme:

I keep saying that managed blocking will perform “some pumping” when
called on an STA thread. Wouldn’t it be great to know exactly what
will get pumped? Unfortunately, pumping is a black art which is
beyond mortal comprehension. On Win2000 and up, we simply delegate to
OLE32’s CoWaitForMultipleHandles service.

This indicates the CLR uses CoWaitForMultipleHandles internally for STA threads. Further, the MSDN docs for COWAIT_DISPATCH_WINDOW_MESSAGES flag mention this:

… in STA is only a small set of special-cased messages dispatched.

I did some research on that, but could not get to pump the WM_TEST from your sample code with CoWaitForMultipleHandles, we discussed that in the comments to your question. My understanding is, the aforementioned small set of special-cased messages is really limited to some COM marshaller-specific messages, and doesn’t include any regular general-purpose messages like your WM_TEST.

So, to answer your question:

… Should I implemented a custom synchronization context, which would
explicitly pump messages with CoWaitForMultipleHandles, and install it
on each STA thread started by StaTaskScheduler?

Yes, I believe that creating a custom synchronization context and overriding SynchronizationContext.Wait is indeed the right solution.

However, you should avoid using CoWaitForMultipleHandles, and use MsgWaitForMultipleObjectsEx instead. If MsgWaitForMultipleObjectsEx indicates there’s a pending message in the queue, you should manually pump it with PeekMessage(PM_REMOVE) and DispatchMessage. Then you should continue waiting for the handles, all inside the same SynchronizationContext.Wait call.

Note there’s a subtle but important difference between MsgWaitForMultipleObjectsEx and MsgWaitForMultipleObjects. The latter doesn’t return and keeps blocking, if there’s a message already seen in the queue (e.g., with PeekMessage(PM_NOREMOVE) or GetQueueStatus), but not removed. That’s not good for pumping, because your COM objects might be using something like PeekMessage to inspect the message queue. That might later cause MsgWaitForMultipleObjects to block when not expected.

OTOH, MsgWaitForMultipleObjectsEx with MWMO_INPUTAVAILABLE flag doesn’t have such shortcoming, and would return in this case.

A while ago I created a custom version of StaTaskScheduler (available here as ThreadAffinityTaskScheduler) in attempt to solve a different problem: maintaining a pool of threads with thread affinity for subsequent await continuations. The thread affinity is vital if you use STA COM objects across multiple awaits. The original StaTaskScheduler exhibits this behavior only when its pool is limited to 1 thread.

So I went ahead and did some more experimenting with your WM_TEST case. Originally, I installed an instance of the standard SynchronizationContext class on the STA thread. The WM_TEST message didn’t get pumped, which was expected.

Then I overridden SynchronizationContext.Wait to just forward it to SynchronizationContext.WaitHelper. It did get called, but still didn’t pump.

Finally, I implemented a full-featured message pump loop, here’s the core part of it:

// the core loop
var msg = new NativeMethods.MSG();
while (true)
    // MsgWaitForMultipleObjectsEx with MWMO_INPUTAVAILABLE returns,
    // even if there's a message already seen but not removed in the message queue
    nativeResult = NativeMethods.MsgWaitForMultipleObjectsEx(
        count, waitHandles,

    if (IsNativeWaitSuccessful(count, nativeResult, out managedResult) || WaitHandle.WaitTimeout == managedResult)
        return managedResult;

    // there is a message, pump and dispatch it
    if (NativeMethods.PeekMessage(out msg, IntPtr.Zero, 0, 0, NativeMethods.PM_REMOVE))
        NativeMethods.TranslateMessage(ref msg);
        NativeMethods.DispatchMessage(ref msg);
    if (hasTimedOut())
        return WaitHandle.WaitTimeout;

This does work, WM_TEST gets pumped. Below is an adapted version of your test:

public static async Task RunAsync()
    using (var staThread = new Noseratio.ThreadAffinity.ThreadWithAffinityContext(staThread: true, pumpMessages: true))
        Console.WriteLine("Initial thread #" + Thread.CurrentThread.ManagedThreadId);
        await staThread.Run(async () =>
            Console.WriteLine("On STA thread #" + Thread.CurrentThread.ManagedThreadId);
            // create a simple Win32 window
            IntPtr hwnd = CreateTestWindow();

            // Post some WM_TEST messages
            Console.WriteLine("Post some WM_TEST messages...");
            NativeMethods.PostMessage(hwnd, NativeMethods.WM_TEST, new IntPtr(1), IntPtr.Zero);
            NativeMethods.PostMessage(hwnd, NativeMethods.WM_TEST, new IntPtr(2), IntPtr.Zero);
            NativeMethods.PostMessage(hwnd, NativeMethods.WM_TEST, new IntPtr(3), IntPtr.Zero);
            Console.WriteLine("Press Enter to continue...");
            await ReadLineAsync();

            Console.WriteLine("After await, thread #" + Thread.CurrentThread.ManagedThreadId);
            Console.WriteLine("Pending messages in the queue: " + (NativeMethods.GetQueueStatus(0x1FF) >> 16 != 0));

            Console.WriteLine("Exiting STA thread #" + Thread.CurrentThread.ManagedThreadId);
        }, CancellationToken.None);
    Console.WriteLine("Current thread #" + Thread.CurrentThread.ManagedThreadId);

The output:

Initial thread #9
On STA thread #10
Post some WM_TEST messages...
Press Enter to continue...
WM_TEST processed: 1
WM_TEST processed: 2
WM_TEST processed: 3

After await, thread #10
Pending messages in the queue: False
Exiting STA thread #10
Current thread #12
Press any key to exit

Note this implementation supports both the thread affinity (it stays on the thread #10 after await) and the message pumping. The full source code contains re-usable parts (ThreadAffinityTaskScheduler and ThreadWithAffinityContext) and is available here as self-contained console app. It hasn’t been thoroughly tested, so use it at your own risk.

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