Class is inaccessible due to its protection level

There was a project that used linked files. I needed to add the method.cs file to that project as a linked file as well, since the FBlock.cs file was there. I've never heard of linked files before, I didn't even know that was possible.

First thing, try a full rebuild. Clean and build (or just use rebuild). Every once in a long while that resolves bizarre build issues for me.

Next, comment out the rest of the code that is not in your example you have posted. Compile. Does that work?

If so, start adding segments back until one breaks it.

If not, make all the classes public and try again.

If that still fails, maybe try putting the trimmed down classes in the same file and rebuilding. At that point, there would be absolutely no reason for access issues. If that still fails, take up carpentry.