FAQ 34.36 What are component categories?

Component categories are a COM mechanism that allows a control to register the types of services it provides. Although the following discussion is phrased in terms of controls and control containers, it applies to any COM component and any COM caller.

Component categories are particularly valuable for control containers since it allows them to determine the capabilities of a control without having to create the control and enter into a lengthy discovery process involving querying the components for the interfaces it supports.

Every component category has a unique category identifier or CATID (which is a GUID) and a list of interfaces a control must implement before it can belong to that component category. Any control that implements all the interfaces defined for a particular component category is then able to register itself as belonging to and supporting the component category.

This permits control containers to look up the component categories for a particular control in the system registry and immediately determine whether or not it provides the services the control container requires so that the control container can adapt its behavior as necessary.