Lost some time over the following bug in Visual Studio 2005, so hopefully this post will prevent someone else doing the same.
When you manually create an App_Code folder in a VS2005 project, and you add a class to it, it probably won’t show up in IntelliSense. Don’t worry, it’s not you, it’s MS.
Every class you add to that folder, will have its Build Action property set to ‘Content’. Change this to ‘Compile’ and problem solved.
I suppose MS will think of this as a feature, because of the fact you created the folder manually, and like the Global Resources folder, might contain mostly icons, images, etc… Then ‘Content’ would be right. But then if they recognize the ‘App_Code’ folder automatically (VS even sets the code-icon for it), they might as well put the default Build Action property to ‘Compile’, dang it.
I do not know if this also occurs with a brand new project, to which App_Code was added on the moment of creation.