There are two main types of servers, in-process and out-of-process. In-process servers are implemented in a dynamic linked library (DLL), and out-of-process servers are implemented in an executable file (EXE). Out-of-process servers can reside either on the local machine or on a remote machine. In addition, COM provides a mechanism that allows an in-process server (a DLL) to run in a surrogate EXE process to gain the advantage of being able to run the process on a remote machine. For more information, see DLL Surrogates.

The COM programming model and constructs have now been extended so that COM clients and servers can work together across the network, not just within a given machine. This enables existing applications to interact with new applications and with each other across networks with proper administration, and new applications can be written to take advantage of networking features.

COM client applications do not need to be aware of how server objects are packaged, whether they are packaged as in-process objects (in DLLs) or as local or remote objects (in EXEs). Distributed COM further allows objects to be packaged as Microsoft Windows NT or Microsoft Windows 2000 Services, synchronizing COM with the rich administrative and system-integration capabilities of Windows NT and Windows 2000.

Note:
Throughout this documentation the acronym COM is used in preference to DCOM. This is because DCOM is not separate—it is just COM with a longer wire. In cases where what is being described is specifically a remote operation, the term distributed COM is used.

COM is designed to make it possible to add the support for location transparency that extends across a network. It allows applications written for single machines to run across a network and provides features that extend these capabilities and add to the security necessary in a network. (For more information, see Security in COM.)

COM specifies a mechanism by which the class code can be used by many different applications.

Concepts