Windows Programming/COM and ActiveX

People familiar with Windows have doubtless heard a number of different terms used, such as COM, DCOM, COM+, ActiveX, OLE, etc. What precisely are all these things, and how are they related?

Introduction to COM

Component Object Model (COM) is a binary-interface standard for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in a large range of programming languages. The term COM is often used in the Microsoft software development industry as an umbrella term that encompasses the OLE, OLE Automation, ActiveX, COM+ and DCOM technologies. COM essentially lays a framework for classes to be created in any language, and to be instantiated into any other languages. A class may be written in C++, and called from VisualBasic, C# etc. Essentially, COM defines an interface that all languages must follow, if they want to participate in COM programs.

COM Libraries

COM classes, written in a compilable language (such as C++ or VisualBasic) may be compiled into DLL libraries. DLL libraries that contain COM classes are not compatible with DLL libraries written in C that contain regular C functions. However, DLL files may be compiled such that they contain both standard C functions, and COM classes.

Files that contain COM classes are referred to as "COM modules", or "COM Components". Just like with other dynamic linking applications, Windows will manage the COM interface between components automatically, so long as all the modules properly use the COM specifications.

UUID

The COM system relies on numerical identifiers known as Universally Unique Identifiers (UUID). A UUID is a 128 bit number that is essentially guaranteed to be unique COM identifier throughout the entire world. The UUID number is a hash value based on the MAC address of your primary network card (if your computer has one) and the real-time clock value of your processor. This means that the only way to generate 2 UUIDs that are identical would be to generate both at exactly the same time on exactly the same computer. Thankfully, this is impossible to do.

COM components are given a UUID number, so that they can be differentiated from all other COM components, and any given COM module can be identified by a single numeric identifier.