FAQ 34.27 How does Invoke accomplish all of this?

FAQ 34.27 How does Invoke accomplish all of this?

IDispatch::Invoke declares a very general interface that has a large number of formal arguments that allow the caller to specify a wide range of parameters including the DISPID for the operation, the type of operation (method call versus property get versus property put), the return value, exception information, and so on. The Automation object's implementation of IDispatch::Invoke must unpackage these parameters, call the method or access the property, and handle any errors that occur. When the property or method returns, the Automation object passes its return value back to the caller through another parameter of IDispatch::Invoke.

Invoke is similar in spirit to printf: callers pass a bunch of information, and the implementation of IDispatch::Invoke (like printf) ends up interpreting the incoming parameters to decide what to do. This makes IDispatch:: Invoke capable of doing almost anything.

After IDispatch::Invoke interprets the parameters passed to it, it calls a method defined by the server in this way IDispatch::Invoke does nothing. All IDispatch::Invoke does is look up the right routine, and that routine performs the useful work.

Notice that dispatch interfaces reverse the usual notions associated with COM interfaces. COM interfaces tend to have a small set of well-defined methods that are semantically related to one another. Dispatch interfaces tend to group together a large number of methods that are not necessarily related and funnel them through the IDispatch interface.