Thread Safety

...XAudio2 might be beholden to the monster that is COM's threading model.

XAudio 2.7

• CLSID_XAudio2 → HKEY_CLASSES_ROOT\CLSID\{5a508685-a254-4fba-9b82-9a24b00306af}\InProcServer32\ThreadingModel = Both
  • Resulting COM objects can only be accessed from within the same apartment (means we can't Send from MTA → STA)
  • If we can forcibly create the class in the neutral apartment, that might suffice to make things Sendable?

XAudio 2.9

• No CLSID, and creation can succeed without COM being initialized (although doing much still requires COM init)
• It's possible to create and use IXAudio2 s from both single threaded and multi-threaded apartments
• No thread safety hinting interfaces are implemented (e.g. IAgileObject, IMarshal, INoMarshal, ...)

References

The Old New Thing:

• Neutral Apartments (microsoft.com)
• Threading Model Attribute (microsoft.com)
• How do I get a foothold in the neutral apartment? (The Old New Thing)
• How do you get into a context via IContext­Callback::Context­Callback? (The Old New Thing)
• Understanding and Using COM Threading Models (microsoft.com)
• Processes, Threads, and Apartments (microsoft.com)
• Agile objects in C++/WinRT (microsoft.com)
• Understanding The COM Single-Threaded Apartment Part 1 (codeproject.com)
• Single-Threaded Apartments (microsoft.com) ≈ !Send + !Sync
• Multithreaded Apartments (microsoft.com) ≈ Send + Sync, but only within MTA threads (e.g. can't send MTA objects to STA threads nor call their methods.)