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

None of the previous solutions worked for me as the project I was attempting to add files to was a "Web Site Project"(WSP). When selecting the project in the solution explorer the "Project" menu item converted to "Website" and there was no "Show all Files" option.

This is because for WSP(link):

"all files are already shown in the Solution Explorer."

Except in my case they weren't... I resolved the issue by simply closing and reopening Visual Studio 2015(VS) and reloading the project. I think VS only detects the addition of new files when the Web Site is re-loaded, not when those files are added outside of VS.