Dispatch Interfaces

Automation controllers are clients that use the COM IDispatch interface to access the COM server objects. The controller must first create the object, then query the object's IUnknown interface for a pointer to its IDispatch interface. IDispatch keeps track of methods and properties internally by a dispatch identifier (dispID), which is a unique identification number for an interface member. Through IDispatch, a controller retrieves the object's type information for the dispatch interface and then maps interface member names to specific dispIDs. These dispIDs are available at runtime, and controllers get them by calling the IDispatch method,GetIDsOfNames.

Once it has the dispID, the controller can then call the IDispatch method, Invoke, to execute the appropriate code (property or method), packaging the parameters for the property or method into one of the Invoke parameters. Invoke has a fixed compile-time signature that allows it to accept any number of arguments when calling an interface method.

The Automation object's implementation of Invoke must then unpackage the parameters, call the property or method, and be prepared to handle any errors that occur. When the property or method returns, the object passes its return value back to the controller.

This is called late binding because the controller binds to the property or method at runtime rather than at compile time.