When you add an existing item in a project, Visual Studio automatically create a new copy of the file. This is usually what you want but sometimes you simply want to add a file as a link in order to reuse it and avoid to manage and maintain two different copies.
The trick is to use an “hidden” drop down menu in the “Add Existing Item” dialog.
Visual Studio is clever enough to show you the code in grey based on where you open the file.
Great solution !