If you want to add the
cs files from one Visual Studio solution (or project) into another, without duplicating the
cs files or moving them over, you can simply use the
Add > Existing file.. option, making sure that instead of selecting the files and clicking
Add (which would copy the files over to your project folder duplicating them) you click on the small arrow and click
Add As Link. The files will be linked to your project as a reference to the other project, and editing them will change their code for both Visual Studio projects.