CODEWITHSUNDEEP
c++multithreadingwinapicom
riverofwind
riverofwind

Reputation: 567

Use COM STA or MTA Without Shared COM Objects?

This thread is great at explaining STA vs MTA for COM, however it doesn't address how to code for such or when to use one or the other, and only just discusses technicalities about COM apartments being used by thread-safe objects or not. I'd be willing to bet most users just want to know how to use the Win API through COM without any COM objects shared among multiple threads.

Should you use STA always if your COM objects aren't shared among threads, and your code makes use of multiple threads or a single thread each with its own COM object instances, none of the objects shared? Does it depend on the object you are using? If you don't always use STA for such when do you use MTA? Do you ever need a message pump in this case?

In my case I use the Task Scheduler API (ITaskService) and the Shell Links/Create Shortcut API (IShellLink) from the main GUI thread (using the Qt Framework), and the File Operation API (IFileOperation) and the Volume Shadow Copy Service API from a worker thread.

I call CoInitializeEx(NULL, COINIT_APARTMENTTHREADED); from each thread before initializing and using the COM objects and CoUninitialize(); after. Is this proper usage of COM? Would the same apply using COM objects from the main GUI thread without a worker thread?

Upvotes: 2

Views: 620

Answers (1)

selbie
selbie

Reputation: 104514

For making outbound COM calls to objects that you instantiated via CoCreateInstance, STA should be good enough, and is almost a must for your GUI thread (the one that has a GetMessage/DispatchMessage loop).

MTA starts to become relevant when hosting your own thread safe COM objects that are expected to be invoked from other processes.

The documentation for IFileOperation states this:

IFileOperation can only be applied in a single-threaded apartment (STA) situation. It cannot be used for a multithreaded apartment (MTA) situation. For MTA, you still must use SHFileOperation.

See all, this link: INFO: Calling Shell Functions and Interfaces from a Multithreaded Apartment

I suspect what the documentation is really trying to say is this:

  1. The class implementing IFileOperation is not thread safe
  2. It's ThreadingModel is declared "apartment" in the registry and will incur marhsalling overhead if accessed from an MTA thread.

On our application, have used ITaskScheduler on the main STA thread. And we use IFileOperation on a background STA thread that has its own message pump.

Some other links that I think are very useful:

https://support.microsoft.com/en-us/help/150777/info-descriptions-and-workings-of-ole-threading-models

https://devblogs.microsoft.com/oldnewthing/?p=22603

Upvotes: 2

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