ArcGIS Engine Developer Guide

THE MICROSOFT COMPONENT OBJECT MODEL
COM allows these objects to be reused at a binary level, meaning that third-party
developers do not require access to source code, header files, or object libraries to
extend the system, even at the lowest level.
COMPONENTS, OBJECTS, CLIENTS, AND SERVERS
Different texts use the terms components, objects, clients, and servers to mean
different things. (To add to the confusion, various texts refer to the same thing
using all these terms.) Therefore, it is worthwhile to define some terminology.
COM is a client/server architecture. The server (or object) provides some functionality,
and the client uses that functionality. COM facilitates the communication
between the client and the object. An object can, at the same time, be a
server to a client and a client of some other object’s services.
Objects are instances of COM classes that
make services available for use by a client.
Hence, it is normal to talk of clients and objects
instead of clients and servers. These objects are
often referred to as COM objects and component
objects. This book will refer to them simply
as objects.
Client and server
COM in-process server
myDLL
COM
objects
COM out-of-process server
process space
myEXE
COM
object
s
client
server
MyComputer
Objects inside an out-of-process server are
accessed by COM-supplied proxy objects, which
make access transparent to the client.
process space
myEXE
COM
objects
proxy
object
process space
client
server
MyComputer
Objects inside an in-process server are accessed
directly by their clients.
COM+ server
process space
yourEXE
proxy
object
yourDLL
process space
yourEXE
client
server
MyComputer
YourComputer
Objects inside an out-of-process server are
accessed by COM-supplied proxy objects, which
make access transparent to the client. The COM
runtime handles the remoting layer.
Client
VBApp.exe
Client/Server
ArcMap.exe
Server
Map.dll
The client and its servers can exist in the same process or in a different process
space. In-process servers are packaged in DLL form, and these DLLs are loaded
into the client’s address space when the client first accesses the server. Out-ofprocess
servers are packaged in executables (EXE) and run in their own address
space. COM makes the differences transparent to the client.
When creating COM objects, the developer must be aware of the type of server
that the objects will reside in, but if the creator of the object has implemented
them correctly, the packaging does not affect the use of the objects by the client.
There are pros and cons to each method of packaging that are symmetrically
opposite. DLLs are faster to load into memory, and calling a DLL function is
faster. EXEs, on the other hand, provide a more robust solution (if the server
fails, the client will not crash), and security is better handled since the server has
its own security context.
In a distributed system, EXEs are more flexible, and it does not matter if the
server has a different byte ordering from the client. The majority of ArcObjects
servers are packaged as in-process servers (DLLs). Later, you will see the performance
benefits associated with in-process servers.
In a COM system, the client, or user of functionality, is completely isolated from
the provider of that functionality, the object. All the client needs to know is that
the functionality is available; with this knowledge, the client can make method
calls to the object and expect the object to honor them. In this way, COM is said
to act as a contract between client and object. If the object breaks that contract,
the behavior of the system will be unspecified. In this way, COM development is
based on trust between the implementer and the user of functionality.
In the ArcGIS applications, there are many objects that provide, via their interfaces,
thousands of properties and methods. When you use the ESRI object
libraries, you can assume that all these properties and interfaces have been fully
implemented, and if they are present on the object diagrams, they are there to
use.
Chapter 4 • Developer environments • 59