How do I find files that are missing from visual studio projects?

Question

Due to issues with merging etc, we have lots of project files that don’t contain all source code files that are within their folders.

Before I write a little tool, that

Answer 1

If anyone has the error File Not Included the first response dated 2011 still works on VS 2017 Version 15.8.5 on .NET Framework Version 4.7.03056 Windows 10 with latest updates. Closing and reopening VS shows all files.