When To Change a GUID on a Type Library

Question

I know that when you add/change/remove methods in a COM interface you're supposed to change the interface/coclass GUID but what about type libraries. When should you change the type library's GUID? Do you change it if a GUID inside the type library has changed? Or should you only change it when something that doesn't have its own GUID within the type library changes.

Answer 1

Here's the basic principal, COM interfaces and Type Libraries should be immutable (don't ever change). If you change one item inside a COM interface it needs to be a completely separate entity from the previous version. The only way to do this is to change the GUID for every interface in the library and the GUID for the type library itself. It's also a good idea (for your own personal sanity) to change the name of the typelib as well.

Ideally you shouldn't ever change a COM interface. Instead create a new derived COM interface and publish in a new type library.

Answer 2

Hi there you both,

I've got a similar question.

I had an original control with CLSID_A that implemented interface IID_A in some 1.0 type library with GUID_A

Later on, I decided to add a new interface to the original control. It would then implement both IID_A and IID_B interfaces. I figured that I should probably keep the same CLSID but didn’t knew much what to do with the typelib itself. I was mostly doing VC++ programmatic-by-the-book stuff which involved QueryInterface and didn’t cared much about versioning and typelib. You wanted to create an object with a specific CLSID, you just asked CoCreated instance...and then Queried interface for potential support of the new interface...

Now when I get into fancier environments like LabVIEW or design-time drop-in development environments like Microsoft .NET, MFC stuff that seems to break.

You are mentioning in your answer to change all of the GUID. Is the whole paradigm of adapting an application based on available functionality dead, that a newer application could still use its basic functionality with the older version of a control? Maybe I didn’t catch the later wave that is: No point in adapting an application to run using old control version, it simply requires a specific control version. That would be reason M$ also came out with the ASSEMBLY thing.