cannot change GUID for class 'class_name'
You attempted to change the class ID with __declspec(uuid).
For example, the following code generates C3121:
[emitidl]; [module(name="MyLibrary")]; [coclass, uuid="00000000-0000-0000-0000-111111111111"] class __declspec(uuid("00000000-0000-0000-0000-111111111112")) A { }; void main() {}