Visual Studio Class Library - Not Compiling Imported CS Files

I recently upgraded a web site project into a web application project and split the business logic into a new class library. The classes already existed within the web project so I copied the files over to the new library. When I built the application Visual Studio said it built successfully however when inspecting the library I couldn't see any of the imported classes, only newly create ones.

The fix to get the classes included (compiled) in the new class library was to select the ".cs" file and in the Visual Studio Properties pane, make sure the "Build Action" property is set to "Compile". Hopefully this will save someone some time.