ASP.NET code in app_code files compile but "Go to definition" doesn't work

PROBLEM: Created some class files in the App_Code folder of an asp.net web application - compiled and there were no errors, but "Go to definition" (F12) wasn't working on some of the classes etc.

SOLUTION: Really easy solution for a beginner's problem - ensure the "Build Action" property of your classes is set to "Compile", not "Content" (it might have defaulted to "Content" because it was a web application not a website (I was just following the App_Code folder name as a convention)).