Almost every time I'm working on an ASP.NET project I create a Visual Studio solution that consists of at least two projects: the web project and a class library. The first one is obviously needed. As for the second, what I put there include all kinds of ASP.NET related classes such as the BasePage class or just custom controls. This makes it easy to use those controls since Visual Studio (usually) automatically adds them to the Toolbox.
There are times however, when I need to make something quick. Perhaps a small project to test a new idea. In those cases, I rarely go for the 2+ projects solution layout since Visual Studio is standing in my way a bit - i.e. by hiding the possibility of adding new project to an already existing Web Site project (solution node is not visible on the tree view in Solution Explorer, but it is still possible to add the project using File menu). So I end up with only one project and the App_Code folder.
Working with App_Code is not a problem as long as you don't need to make some configuration in the Web.config file. Some elements require you to specify a type name. Normally in such a case, you need to provide a valid type format name in a form of <typename>,<assemblyname>. As long as you know the assembly name it's OK, but wait! There is no explicit assembly for App_Code. There is however an assembly behind the App_Code directory. It is simply called App_Code!
That said, it is usually enough to just specify the type name, without assembly name, for types defined in the App_Code directory.
So what about controls? In order to work with a control defined in App_Code directory we need to either register it on a page or in a Web.config file. There is however one issue. In order to register a control in a Web.config we need a namespace. By default however, neither pages nor classes added to an App_Code folder have a namespace. This is only a minor inconvenience because ASP.NET does not prevent us from adding a missing namespace declaration to a class in an App_Code directory. By adding such a namespace to a class representing ASP.NET Control, we can then add a registration in a Web.config file like the following one:
<add namespace="MyControls" tagPrefix="m"/>
Notice that assembly attribute is not required.
For working with code, there is also a special class that helps with issues arising from the fact that there is no explicitly assembly nor a namespace for types in an App_Code directory. If at any time, there is a need to work with a type object of a class from the mentioned directory, .NET framework offers us a class called BuildManager which defines a GetType. Using this method it is possible to get an instance of a type object by providing just a name of a class defined in the App_Code directory.