COM: calling from other thread causes crashes, how to make it run on the same thread?

Question

I am doing a BHO (extension for IE) that receives events on other thread. When I access the DOM from that other thread, IE crashes. Is it possible to make the DOM accessed from the same thread as the main BHO thread so that it does not crash?

It seems like a general COM multithreading problem, which I don't understand much.

Answer 1

Look into using CoMarshalInterface or CoMarshalInterThreadInterfaceInStream

These will give you a wrapped interface to an STA COM object that is thread safe.

Answer 2

I don't know much about IE extensions, but it sounds like some COM object needs to be marked a Single Threaded Apartment, so that the COM runtime system ensures that it is run on the same thread which called it initially. If you can't alter the other object, you could probably route your calls to the DOM through a separate COM object marked as STA to achieve the same effect. Hope this helps... I know a bit about COM multithreading, but not much about IE extensions.

Answer 3

ah, fun fun fun multithreading with COM.

Gerald's answer looks right if you want to transfer an interface pointer from one thread to another exactly once. I've found that the GIT (global interface table) is a big help for this kind of thing if you're in a multithreaded system... basically you don't keep around interface pointers but rather DWORD cookies used by the GIT to get an appropriately-marshaled interface pointer for whatever thread you are using it. (you have to register the object in question with the GIT first, and unregister it later when you are done or your object is finished)

Answer 4

Be careful though. Performance can become a serious issue.

If you're just playing around to learn about BHOs, you can use the STA to make your ::SetSite() implementing object operate as if it were single threaded (this allows you to let other threads pull your BHO's pointer out of the GlobalInterfaceTable as @JasonS mentions.

If you're doing something that is expected to be part of a product I highly recommend you very carefully reconsider going MTA everywhere you can and handling the concurrency and thread safety issues yourself. In this case you would only need to ensure that the threads inter-operating with your BHO COM object, were themselves, initialized for COM.

For example, if you want to monitor incoming/outgoing data of website looking for things (either dangerous or sensitive) - then you do NOT want to force all of those threads down the throat of an STA object because, using Yahoo as an example, more than 30 requests will launch and your BHO will start locking up IE.