Accessing an Object by Using IDispatch

ActiveX clients can use the IDispatch interface to access objects that implement the interface. The client must first create the object, and then query the object's IUnknown interface for a pointer to its IDispatch interface.

Although programmers might know objects, methods, and properties by name, IDispatch keeps track of them internally with a number called the dispatch identifier (DISPID). Before an ActiveX client can access a property or method, it must have the DISPID that maps to the name of the member.

With the DISPID, a client can call the member IDispatch::Invoke to access the property or invoke the method, and then package the parameters for the property or method into one of the Invoke parameters.

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

DISPIDs are available at run time, and, in some circumstances, at compile time. At run time, clients get DISPIDs by calling the IDispatch::GetIDsOfNames function. This is called late binding because the controller binds to the property or method at run time instead of compile time.

The DISPID of each property or method is fixed, and is part of the object's type description. If the object is described in a type library, an ActiveX client can read the DISPIDs from the type library at compile time, and avoid calling IDispatch::GetIDsOfNames at run time. This is called ID binding. Because it requires only one call to IDispatch (the call to Invoke) at run time, rather than the two calls required by late binding, it is generally about twice as fast. Late-binding clients can improve performance by caching DISPIDs after retrieving them, so that IDispatch::GetIDsOfNames is called only once for each property or method.

 

 

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