A different way to reproduce this symptom:
I added a new project to an existing solution and the build output reported errors like "the type or namespace name X could not be found..." but no errors were presented in the error list.
The classes I referred to were in other projects (that were referenced) in the same solution.
It turned out that I by mistake had selected an earlier version of .net in the new project. Once I changed to the same version as the other projects, everything built without any errors.