From other answers to this same question, it seems that this error is secondary to the real problem which is a missing source file. I am not using any source control. I have looked through my entire project and don't see any files with a warning icon, indicating it's missing. If there IS a file missing, what is it missing from?
How can I find the "missing" file? Isn't there any way of resetting this? My whole project is stuck on this.
I had this too. Close VS and reopen. Don't compile. The errors list should tell you which file is missing.