Why does "Add IDL Method" on an Interface add the method to the Module as well as to the CoClass?

Question

This question is about using Visual Studio 2019 for building an out-of-process COM server using ATL. (I have done this before in Borland but this is my first time using MSVC).

I created a project called MyObjectsProject with ATL Project wizard. This created the module class CMyObjectsProjectModule : public ATL::CAtlExeModuleT<CMyObjectsProjectModule> in file MyObjectsProject.cpp .

Then I added a simple object MyObject via "Add > ATL Simple Object" as described in the documentation . This created files MyObject.cpp and MyObject.h containing an interface IMyObject and CoClass CMyObject. So far so good.

I go to IMyObject in the Class View, and right-click "Add > Method" and give it some details, then it adds the method declaration to CMyObject in MyObject.h and an empty definition to MyObject.cpp as CMyObject::method_name() . So far, so good (again).

However at the same time, it added the method declaration and definition to MyObjectsProject.cpp as a class member of CMyObjectsProjectModule. Furthermore:

• The CMyObjectsProjectModule::method_name() version is never called -- when I use the COM server via a client, and call IMyObject::method_name() on an instance of MyObject, it executes the version defined as CMyObject::method_name() as expected.
• I can delete the method declaration and definition from CMyObjectsProjectModule and there are no errors.

The same thing happens if adding a Property. Also, the "Completing operation..." takes about 45-50 seconds to run.

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My question is: What is the reason behind the method being added to the Module as well as to the CoClass , and when would that version of the Method be called? (Or is this just a bug and it is not supposed to be added to the module at all?)

In the Borland IDE , the similar function wouldn't add methods to the Module.

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EDIT: Originally posted the question with the project called MyProject , however the problem only occurs when the project is called MyObjectsProject . I used different names in the question originally from what I observed the problem with, but have now edited the question and reproduced using the exact names in the question.

Answer 1

The method is not supposed to be added to the Module, and it is not supposed to take 45-50 seconds to add the method.

The problem seems to occur whenever the CoClass name is an initial substring of the Module name.

For example it occurs for CMyObject with module CMyObjectsProjectModule, but does not occur for CMyObject with a module of CMyProjectModule.

When the problem is occurring, the Class View for CMyObject > Derived Types > shows derived types CMyObject and CMyObjectsProjectModule.

I guess it is "deducing" a derived type relationship between two types where one is an initial substring of the other; and furthermore, that the behaviour of the "Add Method" decides to add the method to everything that it thinks is a derived type of the CoClass having the method added to.

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Conclusion: This seems to be a bug (or at least, questionable design) in the IDE; and to work around it, you can rename the Module to begin with some unique substring such that none of the CoClasses will coincide with initial substrings of the Module.

The Class Designer tool does not show the two types as derived (i.e. that tool doesn't appear to use the same heuristic as the Class View window does for deciding if one type is derived from another)