How Can StaTaskScheduler Be Used Safely with STA Threads and Legacy COM Objects to Avoid Deadlocks?

StaTaskScheduler and Message Pumping on STA Threads: A Closer Look

The StaTaskScheduler, a component of Parallel Extensions Extras, is intended for task scheduling on both MTA and STA threads. While its documentation implies that blocking TPL operations on STA threads utilize a message-pumping mechanism to prevent deadlocks, this isn't always the case.

Specifically, when working with legacy STA COM objects, blocking operations within BlockingCollection<Task> often fail to trigger message pumping. This omission can result in deadlocks when executed on an STA thread.

A more robust solution is to employ a custom synchronization context:

• Create a Custom Synchronization Context:

  • Override the SynchronizationContext.Wait method to explicitly manage message pumping.
  • Leverage MsgWaitForMultipleObjectsEx with the MWMO_INPUTAVAILABLE flag. This allows message retrieval after a signal without blocking indefinitely.
  • Manually pump and dispatch messages within the SynchronizationContext.Wait implementation.

The ThreadAffinityTaskScheduler example demonstrates a solution that addresses this deadlock issue while preserving thread affinity for STA COM objects across multiple await continuations. Its implementation ensures proper WM_TEST message pumping, thereby preventing deadlocks in the message queue.

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