Visual Studio caches the project file and imported
Today I found this one out the hard way:
Editing Loaded Project FilesVisual Studio caches the content of project files and files imported by project files. If you edit a loaded project file, Visual Studio will automatically prompt you to reload the project so that the changes take effect. However if you edit a file imported by a loaded project, there will be no reload prompt and you must unload and reload the project manually to make the changes take effect.
Written on February 25, 2010