This has gotten a lot better, but for a while invalid compiler errors were a big problem with the. If you have errors that show under the Build Only dropdown, then the issue isn't Intellisense. Well, usually it's me who's wrong, not the compiler but on occasion I get to be right and the compiler is really wrong. I have a standard set of steps I tend to go through when I get compiler errors that are wrong. I haven't run into this problem very frequently but when it does happen it's usually quite vexing resulting (for me at least) in a flurry of deleting output folders. suo file contains cached IntelliSense data and once that file is off, no amount of recompilation or clearing the project is going to help. suo file is a nice quick thing to try first.

When VS acts up and reports whacky errors that seem wrong, the burning down the. suo file which is responsible for cached IntelliSense and also some cached Debug data. In these older versions you can fix Intellisense issues by deleting the Solution's. Older versions of Visual Studio (prior to VS 2015) didn't have a separate folder and dumped that same information into files in the solution's root folder. It's safe to delete this folder - Visual Studio recreates it when it's missing. That folder also holds Web site configuration data for Web projects and a few other things. 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:

Studio one 3 file broken