How Can I Resolve Deadlocks When Using StaTaskScheduler with Legacy STA COM Objects?

StaTaskScheduler Deadlocks and STA Thread Message Handling

Problem:

Using StaTaskScheduler with legacy STA COM objects can lead to deadlocks. This occurs because blocking waits within StaTaskScheduler don't automatically pump messages, resulting in a stalled message loop.

Solution 1: Custom Synchronization Context

The solution involves a custom synchronization context that actively pumps messages using MsgWaitForMultipleObjectsEx. This context overrides the Wait method to use WaitHelper and incorporates a message pump loop. Here's a breakdown:

1. Override SynchronizationContext.Wait: Delegate the Wait call to SynchronizationContext.WaitHelper.
2. Implement the Message Pump:
   • Employ MsgWaitForMultipleObjectsEx to detect pending messages, including those already processed.
   • If messages are present, use PeekMessage and DispatchMessage to process them.
   • Continue waiting if the timeout hasn't elapsed.

Solution 2: ThreadAffinityTaskScheduler

Alternatively, a custom StaTaskScheduler, called ThreadAffinityTaskScheduler, provides a built-in message pump and maintains thread affinity for subsequent await operations. Here's the process:

1. Instantiate ThreadWithAffinityContext, a context managing both thread affinity and message pumping.
2. Launch the STA thread using ThreadWithAffinityContext.Run().
3. Execute operations within the STA context, including message posting.
4. Post-await continuations retain thread affinity, and the custom message pump ensures message processing.

Important Notes:

• MsgWaitForMultipleObjectsEx is superior to MsgWaitForMultipleObjects for message pumping because it handles messages already in the queue.
• Implement timeout checks in the custom message pump to prevent infinite loops.
• ThreadAffinityTaskScheduler offers a streamlined solution when both thread affinity and message pumping are necessary within STA contexts.

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