Question

I have this old C++ COM component. I took the latest code base, built it and found that one of the properties has become lower case. For example, in the pre-compiled dll i have a property “Type”, but when building from source it’s called “type”. The idl shows that the property is called “Type”. So what could possibly be happening here?

Answer 1

You discovered a quirk in the OS stock implementation of ICreateTypeLib, used by practically all tool chains on Windows that can create a type library. It uses a rather crude way to deal with possible problems caused by languages that are not case-sensitive, VB/A being a prominent example.

At issue is the definition of an identifier with one casing, being referenced elsewhere in the type library with another casing. Not a problem at all in, say, VB, big problem when the client programmer uses a case-sensitive language like C# or C++.

The “fix” it uses is to force the casing to be consistent everywhere in the library. Unfortunately it is not very sophisticated about it. Best example is a method declaration earlier in the type library that takes an argument named type. Any identifier named Type in the rest of the type library will now get case-converted to type.

Repairing this problem is easy enough, just change the name of the identifier so it no longer matches. You’ll have to find it, not so easy, best to use Oleview.exe, File > View Typelib command. Copy/paste the decompiled IDL into a text editor and use its Search command.