If in VS.NET you add to the current project an existing file that’s located outside the current project’s directory, the file is first copied in the project’s directory, and then it’s added to the project. However, you may want to share the same source file among multiple projects, so that if you modify the code from one project (to fix a bug, for example) the updated code is immediately available to the other projects. To add a shared file, open the dialog to select an existing file with the Project | Add Existing Item