After talking to an app store support engineer at Apple it’s beginning to look like Monogame apps can’t be signed correctly for the Mac App Store. This is pretty recent, because Apple changed their packaging requirements for Mac .apps only a year ago or so.
Specifically, the following page on the Monogame documentation about how to create a Macintosh .app is no longer correct: Package games for distribution | MonoGame Documentation
The correct way to bundle is listed here: Apple Developer Documentation
DLLs and Dylibs can no longer be stored inside the MacOS directory. DLLs must be stored in Contents/Resources. As far as DYLIB files, those must be placed in a separated directory called Contents/Frameworks. Your probably saying, “Why don’t you just put those files in those new directories and sign the .app?” You can do that and sign it, but the app won’t run anymore because monogame executables expect the DLLs and DYLIBS to be in the same directory as the executable.
I tried just creating a Xamarin Mac app and adding a reference to my monogame project to publish from there as Xamarin packages correctly for the Mac App Store. However, Monogame uses .Net Core and Xamarin uses .Net Framework. Xamarin can’t have a .Net Core project, because it is incompatible with the .Net Framework.
The only recourse I can see is if somebody knows of a way to force Monogame to put the DLL’s in a separate folder called “Contents/Resources” when doing a dotnet -publish… The same goes for the DYLIBS, dotnet -publish… would have to put those in “Contents/Frameworks”
If anyone has a solution or any ideas I’m all ears.