ArcGIS Engine Developer Guide

THE MICROSOFT COMPONENT OBJECT MODEL
The function DllGetClassObject is the function
that makes a DLL a COM DLL. Other functions,
such as DllRegisterServer and
DllUnregisterServer, are nice to have but not
essential for a DLL to function as a COM DLL.
2. SCM looks for the requested objects registry entry by searching on the class ID
(a GUID).
3. DLL is located and loaded into memory. The SCM calls a function within the
DLL called DllGetClassObject, passing the desired class as the first argument.
4. The class object normally implements the interface IClassFactory. The SCM
calls the method CreateInstance on this interface to instantiate the appropriate
object into memory.
5. Finally, the SCM asks the newly created object for the interface that the client
requested and passes that interface back to the client. At this stage, the SCM
drops out of the equation, and the client and object communicate directly.
From the above sequence of steps, it is easy to imagine how changes in the
object’s packaging (DLL versus EXE) make little difference to the client of the
object. COM handles these differences.
AUTOMATION
Automation is the technology used by individual objects or entire applications to
provide access to their encapsulated functionality via a late-bound language.
Commonly, automation is thought of as writing macros, where these macros can
access many applications for a task to be done. ArcObjects, as already stated, does
not support the IDispatch interface; hence, it cannot be used alone by an automation
controller.
Chapter 4 • Developer environments • 69