ArcGIS Engine Developer Guide

THE MICROSOFT COMPONENT OBJECT MODEL
IClassFactory
InterfaceB
IClassFactory
InterfaceA
IUnknown
IUnknown
Class
factory A
COM
object B
InterfaceB
IUnknown
Class
factory B
IUnknown
COM
object A
IUnknown
COM
object B
A server is a binary file that contains all the
code required by one or more COM classes. This
includes both the code that works with COM to
instantiate objects into memory and the code to
perform the methods supported by the objects
contained within the server.
GUIDGEN.EXE is a utility that ships with
Microsoft’s Visual Studio and provides an easy-touse
user interface for generating GUIDs. It can
be found in the directory \Common\Tools.
The acronym GUID is commonly pronounced
“gwid”.
CLASS FACTORY
Within each server there is an object called a class factory that the COM runtime
interacts with to instantiate objects of a particular class. For every corresponding
COM class, there is a class factory. Normally, when a client requests an object
from a server, the appropriate class factory creates a new object and passes out
that object to the client.
SINGLETON OBJECTS
Although this is the normal implementation, it is not the only implementation
possible. The class factory can also create an instance of the object the first time
and, with subsequent calls, pass the same object to clients. This type of implementation
creates what is known as a singleton object since there is only one
instance of the object per process.
GLOBALLY UNIQUE IDENTIFIERS
A distributed system potentially has thousands of interfaces, classes, and servers,
all of which must be referenced when locating and binding clients and objects
together at runtime. Clearly, using human-readable names would lead to the
potential for clashes; hence, COM uses GUIDs, 128-bit numbers that are virtually
guaranteed to be unique in the world. It is possible to generate 10 million GUIDs
per second until the year 5770 A.D., and each one would be unique.
The COM API defines a function that can be used to generate GUIDs; in addition,
all COM-compliant development tools automatically assign GUIDs when
appropriate. GUIDs are the same as Universally Unique Identifiers (UUIDs),
defined by the Open Group’s Distributed Computing Environment (DCE) specification.
Below is a sample GUID in registry format.
{E6BDAA76-4D35-11D0-98BE-00805F7CED21}
COM CLASSES AND INTERFACES
Developing with COM means developing using interfaces, the so-called interfacebased
programming model. All communication between objects is made via their
interfaces. COM interfaces are abstract, meaning there is no implementation
associated with an interface; the code associated with an interface comes from a
class implementation. The interface sets which requests can be made of an object
that chooses to implement the interface.
How an interface is implemented differs among objects. Thus, the objects inherit
the type of interface, not its implementation, which is called type inheritance.
Functionality is modeled abstractly with the interfaces and implemented within a
class implementation. Classes and interfaces are often referred to as the “what”
and “how” of COM. The interface defines what an object can do, and the class
defines how it is done.
COM classes provide the code associated with one or more interfaces, thus encapsulating
the functionality entirely within the class. Two classes can have the same
interface, but they may implement them quite differently. By implementing these
interfaces in this way, COM displays classic object-oriented polymorphic behavior.
COM does not support the concept of multiple inheritance; however, this is
60 • ArcGIS Engine Developer Guide