I have a typelib that describes some interfaces. As some of these interfaces are used as a category, I want to add the category IDs to the typelib. So the question is:
a) how can this be done in Delphi (2007 and up)?
or as an alternative
b) is it possible and advisable to use the interface GUID for the CATID?
For a), I don't know Delphi. But in C++ it's simply adding the interface description to the MIDL file. As you seem to really only use the interface, you probably do not need a CoClass implementation either, so this would suffice already. Basically, how did you add the other interfaces to the typelib? Do it the same way now.
[edit]I've found a short tutorial on using the typelib-editor. Seems that adding interfaces is only a matter of the correct keyword.[/edit]
As for b), I do not see any problem to use interfaces as a category-id, but this question is a bit vague... As the set of possible categories is fixed, and you do not use some inheritance like behavior (at least I think so as you said you use the interface GUID, not the interface itself), why not just use an enumerator? Using interfaces etc. seems like an over-engineered solution, except if the interfaces are really used as such, i.e. providing access to some category-specific behavior.
[edit]As you're talking about ICatRegister et al., I see why you want to use GUIDs. As the category GUIDs are in a different namespace than the COM interface GUIDs, sure, use the same. Your interface GUIDs need to be unique within the same scope as the category-ids anyhow, and having a 1:1 mapping seems a simple model for maintenance, too.[/edit]