That's Visual Studio losing track of some of its internal bookkeeping. It has nothing to do with your code. I've never had a serious problem caused by it.
Still, I have managed to get rid of the warning in the past by simply deleting all of the "internal bookkeeping" files, thereby forcing it to regenerate all of them. Specifically, I deleted all of the files in [project folder]/[project name]/Debug and [project folder]/[project name]/Release. Note that those are different from [project folder]/Debug and [project folder]/Release.