The IDispatch Interface

IDispatch is the heart of Automation. It's fully supported by COM marshaling (that is, Microsoft has already marshaled it for you), as are all the other standard COM interfaces, and it's supported well by the MFC library. At the component end, you need a COM class with an IDispatch interface (plus the prerequisite class factory, of course). At the client end, you use standard COM techniques to obtain an IDispatch pointer. (As you'll see, the MFC library and the wizards take care of a lot of these details for you.)

Remember that Invoke is the principal member function of IDispatch. If you looked up IDispatch::Invoke in the Visual C++ online documentation, you'd see a really ugly set of parameters. Don't worry about those now. The MFC library steps in on both sides of the Invoke call, using a data-driven scheme to call component functions based on dispatch map parameters that you define with macros.

Invoke isn't the only IDispatch member function. Another function your controller might call is GetIDsOfNames. From the VBA programmer's point of view, properties and methods have symbolic names, but C++ programmers prefer more efficient integer indexes. Invoke uses integers to specify properties and methods, so GetIDsOfNames is useful at the start of a program for converting each name to a number if you don't know the index numbers at compile time. You've already seen that IDispatch supports symbolic names for methods. In addition, the interface supports symbolic names for a method's parameters. The GetIDsOfNames function returns those parameter names along with the method name. Unfortunately, the MFC IDispatch implementation doesn't support named parameters.