Content vs Compile

I ran into this problem a few weeks ago while writing a class in one of my ASP.Net applications. For some reason, the Visual Studio 2010 environment considers any class you add to the solution as "content". What content means is that the code doesn't compile. This also means that you can not access the code using intelli-sense nor add references to the class in other parts of your code. It can be extremely frustrating to debug.

To solve the problem, select the class in question and then change "Build Action" in the properties window of that class to "Compile" instead of content.