Although I work on a large MFC based project, often I create small independent projects before converting and integrating that code into our main product. I usually don’t turn on use of MFC until integration of the project because most of the low-level work I’m doing doesn’t use MFC.

After integrating the code, I need to turn on use of MFC in order to integrate with the rest of the product, and this invariably causes linker errors:

error LNK2005: _DllMain@12 already defined in X
fatal error LNK1169: one or more multiply defined symbols found

There is lots of information about how to solve this, but these solutions never solve the problem. For my workflow, the solution is to remove the preprocessor definition

_USRDLL

Hopefully next time, I’ll remember this post and not spend half a day trying to identify the problem.