FAQ 34.06 What are the key features of COM?

Language transparency, location transparency, program to the interface rather than to the implementation, unique naming of classes and interfaces, meta data, and a system registry.

Language transparency: COM defines object interfaces in a manner that is independent of any particular programming language. This means that a caller can use a COM object without knowing or caring what programming language it is implemented in. Conversely, the COM class can be implemented in a variety of programming languages without concern for what programming language will be used by callers.

Location transparency: COM provides the infrastructure so that callers can create and access COM objects no matter whether the COM object is running in the caller's process as an in-process object or running in another process on the same computer or running remotely on another computer.

Interface definition: A COM interface is a set of related methods that have a well-defined contract but no implementation (see FAQ 34.09). The signatures for the methods of a COM interface can be defined using a programming language (such as C++ and C) or the Microsoft Interface Definition Language (MIDL), which is based on the Distributed Computing Environment Interface Definition Language (DCE IDL) specification of the Open Software Foundation.

Unique Naming of Classes and Interfaces: COM classes and interfaces have unique "names" called globally unique identifiers (GUIDs; see FAQ 34.07). Unique names are required so that the names of components being developed by different groups in different organizations and in different parts of the world do not accidentally clash.

Meta data: COM provides support for meta data (that is, information about interfaces and classes) in the form of type libraries (see FAQ 34.29). Type libraries contain machine-readable definitions of COM interfaces and/or COM classes and can be accessed programmatically at runtime.

System Registry: COM stores static information about classes, interfaces, and type libraries in a registry so that it can be looked up at runtime. The information in the registry can be accessed prior to any objects being created. In fact, the registry contains the information necessary for loading COM servers (see FAQ 34.12), locating class objects, and creating COM objects. The registry is a hierarchy of key/value pairs and contains information such as the name of the file containing a COM class and the name of the file that contains a type library that defines a particular COM interface.