I can't find where I found instructions for doing this, but on 4.2, I had 2 entities configured an Interface. The implementation was defined in a file in the DatabaseGeneric\PartialClasses folder which was automatically included in the generic .csproj.

I'm guessing older versions automatically included all files in the PartialClasses folder as I cannot find anything in my llblgenproj defining this addition.

How do I add this file to my project? I'm sure this is simple and my search-fu is just letting me down.

The code generator generates a csproj that doesn't include any additional files, so I think it was manually added to the csproj file. This is ok btw, you can manually add any files you create yourself, like partial classes and place them in subfolders and add them manually as existing files to the csproj in visual studio or rider. The code generator won't remove them the next time you generate code. (as it won't overwrite an existing csproj file but augment it).

In v5.4 we moved the location of csproj files and also added auto-cleanup to the code generator. Please see: https://www.llblgen.com/Documentation/5.9/LLBLGen%20Pro%20RTF/migratingcode.htm#migrating-generated-code-from-v5.3-to-v5.4 and https://www.llblgen.com/Documentation/5.9/Designer/Functionality%20Reference/CodeGenerationAspects.htm#automatic-cleanup-of-outdated-files

Thanks for the clarity! I started to suspect this as I poked around in my commit history.