How do you add an existing form to a new project?

Here's an approach that doesn't involve closing the project and reopening it:

1. Add the 3 existing form files (cs, Designer.cs and resx).
2. Now, exclude the 3 files you just added from the project.
3. Open the Add existing item explorer window and go to your project directory.
4. Select the cs file and Add.
5. Tada. all good

After some more research I discovered the source of the issue. It is correct that all one has to import is the *.cs file. The caveat lies in the project type. If working in a Class Library project, another step must be performed.

1. Add a reference to System.Windows.Forms.dll and System.Drawing.
2. Import the *.cs file

Notes:

A. The files are only properly recognized after I performed these steps and then tried to open the file. Suddenly VS "wakes up" and fixes the files.

B. Order of the steps does not matter. If you already imported *.cs files, just fix the references.

C. If one is missing other references e.g. DevExpress or other 3rd party control imports), the *.cs files will not display properly until this has been resolved.