GUIDs and TypeLibs and Interfaces

Unfortunately, to understand the different compatibility levels, the first place we need to go is deep into the bowels of COM. At its heart, a COM component provides services to its clients by exposing classes. By using these classes, it is possible for clients to create objects and manipulate properties and methods to their benefit. The set of properties and methods that are exposed by a class (the ones that are declared as Public) is called its interface. The information about the interface, including each property (name and data type) and method (name, parameter list and data type) is contained in a type library, also called a TypeLib. For VB components, the TypeLib is stored as a resource in the DLL/OCX file. For C++ components, it is usually be found in a separate file with a .tlb extension.

Now the first time you compile your ActiveX project, each interface (remember that each class has its own set of properties and methods and therefore its own interface) is automatically assigned a Globally Unique Identifier, or GUID. This is a 128-bit number that is used to identify the interface. How unique? Well, you would have to compile your application multiple times a second for well over a century in order to generate a conflicting GUID. This GUID is stored in your registry and acts as a link between the component's interface, the class name and the file that contains the executable code.