Class cannot find another class in the same namespace

It is a strange error, in my VS2015 if I set a file Build Action to anything other than "Compile", I get an error underline on any type for that file.

Anyway the solution here is to verify that the Build Action is set to "Compile", I'm not sure why adding a new file would have set the build action to anything other than "Compile". I've also tested trying to add new files in multiple ways (select a text file template and just name it something.cs), it still sets it as "Compile". You should verify that your VS2015 instance is updated with the latest updates.