ISAPI and COM

Working with COM interfaces under ISAPI requires some consideration of the Internet Information Service thread pool architecture.

IIS maintains a pool of I/O threads that are used to call an ISAPI's callback functions. It is possible that the thread which calls your initialization code in DllMain or GetExtensionVersion will not be the same thread that calls HttpExtensionProc. It is also likely that different threads will call HttpExtensionProc from one request to another.

It is important, then, to note that COM initialization affects the thread in which it is called. For this reason, you should never initialize COM within CoInitialize or CoInitializeEx unless you will be uninitializing it before your callback function returns.

As a result of this architecture, you have three design options to use COM from ISAPI.

• You can do all of the initialization and uninitialization from within HttpExtensionProc for each request.
• You can avoid initializing COM. This will allow your ISAPI filter or extension to run in a multithreaded apartment because IIS initializes one of its threads at start time.
• You can create your own pool of threads and initialize them when the ISAPI extension is loaded.

Because the first option forces initialization on each request (and leaves your code vulnerable to other ISAPI extensions that may have initialized COM on the IIS thread), the last option is preferred.