Visual Studio “Go to definition” disabled or gray out

Question

Visual Studio\'s Go To Definition is disabled and F12 doesn\'t work. Other commands like Alt-F12 may continue working.

Answer 1

This also happens, if Visual Studio has files opened, which are not in the current Solution.

I don't know how I got to this state, where files of a different solution where open as I didn't open them manually, but a quick check of the file path showed that those weren't files of the opened solution.

Therefore, "Go to Definition" was disabled.