CoInitializeEx

CoInitializeEx is a Windows API function that initializes the COM library for use by the calling thread and establishes the thread’s COM apartment model. It must be called before using COM components on a thread that has not yet been initialized, and each successful initialization should be balanced with a corresponding CoUninitialize call.

The function takes two parameters: pvReserved, which is reserved for future use and must be NULL, and dwCoInit, a set of flags that specify the threading model and other options. The primary flags correspond to the thread’s apartment model: COINIT_APARTMENTTHREADED (STA) and COINIT_MULTITHREADED (MTA). Additional flags include COINIT_DISABLE_OLE1DDE and COINIT_SPEED_OVER_MEMORY. The exact flag values are defined in the system headers.

CoInitializeEx returns an HRESULT. S_OK indicates that the COM library was initialized on this thread. S_FALSE indicates the COM library was already initialized on this thread. RPC_E_CHANGED_MODE is returned if the thread is already initialized with a different threading model than requested, in which case the initialization request fails. Other error codes, such as E_OUTOFMEMORY, may also be returned depending on the failure mode.

Usage considerations include selecting the appropriate apartment model for the thread’s purpose: GUI threads that host COM components commonly use COINIT_APARTMENTTHREADED, while background worker threads may use COINIT_MULTITHREADED. If a thread changes its threading model after initialization, it cannot be reinitialized with a different model. Each successful CoInitializeEx call must be matched with a corresponding CoUninitialize call to avoid resource leaks.

Availability and relation to other APIs: CoInitializeEx is part of the Windows API and is implemented in the Ole32 library. It works together with CoUninitialize and the broader COM initialization lifecycle in Windows.