If it is the case that only the IDE indicates that it cannot find included files, but compiling is successful, the issue is simply that IntelliSense is not fully up to date with recent changes. This can happen specifically when including existing projects, in my own experience. Deleting the .sdf file (= IntelliSense database) that is generated in your solution directory forces Visual Studio to regenerate it, so that it is up to date again. Just doing a "clean" will probably do the same thing, but takes more time since everything will be generated again then.