从BackgroundWorker C＃调用的CoInitialize

Question

I have a C# Windows Form application that utilizes a C++ DLL. Within the DLL, I initialize COM:

auto hResult = CoInitialize(NULL); // Initialize COM
if (hResult != S_OK && hResult != S_FALSE) { 
    WSACleanup(); 
    return 1; 
}

When I run the DLL outside of a BackgroundWorker process, everything works fine. If I do though, my application freezes when the DLL is finished. So, I'm attempting to use a BackgroundWorker; but whenever I run the DLL within the DoWork function I am unable to initialize COM.

Could someone explain this, please, and offer any suggestions on how to run my DLL in the BackgroundWorker?

Thank you.

Answer 1

BackgroundWorker uses thread pool threads. .NET thread pool threads are automatically initialized to MTA ( CoInitializeEx(NULL, COINIT_MULTITHREADED) ). Your DLL is trying to initialize the thread to STA ( CoInitialize() ), and that call should be returning RPC_E_CHANGED_MODE . This is a failure.

Typically, I wouldn't initialize COM on the calling thread from within a library. I would consider this an antipattern. Multiple libraries can be used by a single client application, and each of those libraries might (try to) initialize COM. A better design is to have the owner of each thread initialize COM on that thread. Your client application would initialize COM for the main thread and any background threads it owns (.NET does all this for you). Each library would specify (in documentation) the threading/apartment requirements for its entry points (eg "This DLL's FooExport function must be called from an STA thread."). Threads owned by the library would have their apartment state controlled by the library. The only real advantage to calling CoInitialize/Ex from within the library is to try to detect the apartment state that your thread is currently in so that the library's apartment requirements are checked programmatically, but there are some scenarios (neutral-threaded apartments) in which that becomes problematic.

To your scenario:

• If your DLL requires STA, create the background thread manually in your client application, and set the apartment state to STA before starting the thread (see SetApartmentState ). Also consider removing the CoInitialize call in your library.
• If your DLL can use MTA, either remove the CoInitialize call from your DLL, or use CoInitializeEx(NULL, COINIT_MULTITHREADED) .