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COM & .NET Component Services by Juval Lowy

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Cross-Context Manual Marshaling

Cross-context call interception via marshaling is how COM+ provides its component services to your object. A client in a different context cannot access your object directly, even if it has a direct raw pointer to it. Intercepting the call and performing the right service switches requires a proxy and a stub in between. Otherwise, the object executes in the client context, possibly in an ill-suited runtime environment. If the client gets the pointer to your object in one of the following ways:

  • CoCreating the object

  • Querying an object the client already has for additional interfaces

  • Receiving the pointer as a method parameter on a COM interface

Then COM+ will, under the hood, put interceptors (proxys and stubs) in place, to make sure all calls into the object are marshaled. If the client does anything else to obtain the interface pointer, such as retrieve it from a global variable or a static member variable shared among all clients, you have to marshal the pointer manually yourself. Dealing with pooled objects is another situation requiring manual marshaling, as you will see in the next chapter.

Classic COM requires that all cross-apartment calls be marshaled, even when the call is in the same process, to ensure threading model compatibility. The classic COM mechanisms for manually marshaling interface pointers across apartment boundaries have been made context-aware. They are what you should use to marshal interface pointers manually across context ...

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