In-Process Server Threading Issues

Like other servers, in-process servers can be single-threaded, apartment-threaded, or free-threaded. These servers can be used by any OLE client, regardless of the threading model used by that client. For an in-process server, when the threading model of the client and in-process server differ, COM must interpose itself between the client and the object. When an in-process object that supports the single-threaded model is called simultaneously by multiple threads of a client, COM cannot allow the client threads to directly access the object's interface—the object was not designed for such access.Instead, COM must ensure that calls are synchronized and are made only by the client thread that created the object. Therefore, COM creates the object in the client's main apartment and requires all the other client apartments to access the object by using proxies. When a free-threaded apartment (multithreaded apartment model) in a client creates an apartment-threaded in-process server, COM spins up a single-threaded apartment model "host" thread in the client. This host thread will create the object, and the interface pointer will be marshaled back to the client's free-threaded apartment. Similarly, when a single-threaded apartment in an apartment-model client creates a free-threaded in-process server, COM spins up a free-threaded host thread (multithreaded apartment on which the object will be created and then marshaled back to the client single-threaded apartment). COM helps protect access to objects provided by a single-threaded DLL by requiring access from the same client apartment in which they were created. In addition, all of the DLL entry points (like DllGetClassObject and DllCanUnloadNow) and global data should always be accessed by the same apartment. COM creates such objects in the main apartment of the client, giving the main apartment direct access to the object's pointers. Calls from the other apartments use interthread marshaling to go from the proxy to the stub in the main apartment and then to the object. This allows COM to synchronize calls to the object. Interthread calls are slow, so it is recommended that these servers be rewritten to support multiple apartments. single-threaded的全称应该是single-thread-per-process。