In the case of passing an object to a newly-created thread, you would do something like this: Note, of course, that the pointer that comes out of IAgileReference::Resolve is valid only in the apartment in which you called IAgileReference::Resolve. What makes the agile reference special is that it is free-threaded: You can share the pointer freely among apartments.³ To gain access to the original interface pointer, call IAgileReference::Resolve.
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You give it an interface accessible to the current apartment, and the RoGetAgileReference function returns you an IAgileReference interface pointer that figures out how to create proxies so you don’t have to. You can use the RoGetAgileReference function. What if I need to marshal an object from one thread to another thread in the same process, and I don’t have a friendly object on the other thread yet? In other words, once you have one object operating across apartments, you can use it to get more objects to operate across apartments. Of course, this creates a chicken-and-egg problem: In order to get my object to another apartment, I need access to an object on that apartment.
It can event retain that pointer (with an appropriate AddRef, of course) to communicate with the original widget even after the method returns. The implementation of IMumble::ColorizeWidget will receive a pointer to the proxy, and it can use the proxy to access the original widget. Just call the method from your originating apartment. For example, if you need to pass a widget to another object, you probably already have a method on that other object called IMumble::ColorizeWidget that takes a widget and some other parameters. The easy way to do this is to take advantage of the parenthetical remark up above: If you already have an interface pointer to an object in another apartment, you can define a marshalable interface that accepts an interface pointer. What if I need to marshal an object to another thread (same process or different process), and I already have access to an object in the destination thread? (And if any of the parameters to the method are themselves objects, then COM needs to create proxies for those objects, too!) Marshaling is a mechanism for creating proxies.² When you invoke a method on the proxy object, the call is routed back to the originating apartment, the method executes on its original apartment, and then the results are routed back to original caller. Since the rules for apartment model objects is that they can be accessed only from the thread on which they were created, you need to do some extra work if you want to access them from another thread: You need to hire a lackey. Free-threaded objects are easier to use, but apartment model objects are much, much easier to write, since you don’t have any of that troublesome multi-threading to deal with. The most common cases are apartment model objects,¹ which can be used only on the thread that they were created on, and free-threaded objects, which can be used from any thread. Incomplete backgrounder on threading models: Each COM object declares how it deals with threads. In this world, the concepts of thread and apartment line up one-to-one, which makes discussion easier. Single-threaded apartments, as you might guess from their name, are apartments that consist of a single thread. The details of apartments aren’t important for today’s discussion, so let’s assume that all the apartments in question are single-threaded apartments (STA). Incomplete backgrounder on apartments: In COM, an apartment is a collection of threads that are treated as equivalent from a COM standpoint. Basically, marshaling is the mechanism by which an object that is accessible to one apartment can be made accessible to another apartment. COM has this concept called “marshaling”, with one L.
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