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A Single-Threaded Apartment (STA) is a thread that was initialized with CoInitialize() or CoInitializeEx (NULL, COINIT_APARTMENTTHREADED). Also, any other threads in the same process that use COM must also call CoInitialize() or CoInitializeEx() to initialize COM for its thread.
If you create a COM object in one STA thread, you cannot pass an interface pointer to another STA thread and call out on that pointer. Since calls to STA objects are supposed to be serialized, COM enforces this by only allowing one thread to call into the STA object (the thread where it was created). If the interface pointer you pass to the second STA thread is a pointer to a proxy, you will get an error HRESULT of 0x8001010E or RPC_E_WRONG_THREAD (the application called an interface that was marshaled for a different thread). If the interface pointer is a direct pointer to the object, COM will not enforce serialization, you will not get the above error, and the interface method call will be made. However, this is still illegal behavior on the part of the client.
You can still call methods on the STA object from a different STA thread as long as you do it through a proxy. A proxy is a copy of the interface that you get via marshaling/unmarshaling. When you make a call through the proxy, COM makes a thread switch and the call ends up executing in the context of the thread where the STA object was created.
There are three ways to marshal/unmarshal an interface to another STA thread:
- CoMarshalInterThreadInterfaceInStream() and CoGetInterfaceAndReleaseStream(): One drawback to using this method is the interface can only be unmarshaled once. That is, if you need access to the same object from several STA threads, this method won't work.
- CoMarshalInterface() and CoUnMarshalInterface(): This method is more flexible since you can marshal the interface once and unmarshal the interface as many times as you like; for example, specifying MSHLFLGS_TABLEWEAK or MSHLFLGS_TABLESTRONG when marshaling the interface. But it also requires more code since you have to create the IStream, set the seek pointer and clean up the marshal packet via CoReleaseMarshalData(). When marshaling pointers to a proxy you must have Windows NT 4.0 Service Pack 4 or later, Windows 98 or Windows 95 with DCOM 1.2 or later installed. Also when using the MSHLFLGS_TABLESTRONG flag you must have Windows 2000 or later.
- Global Interface Table(GIT): The GIT is a COM object that allows you to store an interface in one STA thread and get access to it's proxy in another STA thread. This is only available with Windows NT 4.0 Service Pack 3 or later, Windows 98 or Windows 95 with DCOM 1.1 or later installed.
Article ID: 206076 - Last Review: May 22, 2008 - Revision: 1