suo file that caches intellisense and some debug data. vs folder holds solution related temp data including the. There's usually a simple solution when IntelliSense decides to sleep one off:

Not sure when it arrived but it was in one of the late VS 2015.x updates. Note the Intellisense error drop down is a new feature so you may not see it in older versions of Visual Studio. Notice the drop down in the error list that lets you see Build + Intellisense which, in the case above produces 3 errors, but no errors if I just show Build Only: So IntelliSense sometimes goes off the rails and if you see errors in your project that don't make sense, first check to see if the errors are related to Intellisense. The actual build of the code succeeds, but Intellisense is flagging several classes as missing even though they clearly exist and clearly compile properly. It's always fun when Visual Studio (ie can't recite the alphabet backwards: The code below builds just fine when running through the compiler, but Visual Studio displays Intellisense errors in the Error Window and in the code with underlined squiggles:

Online msvc compiler