The main idea of the Unity Framework is to give us the ability to quickly replace different parts of big application without having to recompile or re-factor our code. Ideally that means adding another assembly to the bin folder or GAC, and certainly that is the right way to add new providers or class implementations. But sometimes you may want to quickly script a provider in order to test a new idea on your staging server or debug a problem without having to deal with all the hassle of adding a new assembly.
Well, the first idea that comes to my mind is to use a class defined in the App_Code folder. The class will be recompiled each time there is a change in the source file so this seems as a perfect candidate for a quick test. But there is one problem. The assemblies for the classes in App_Code folder are dynamic and so we don't know what their name is, and because of that we cannot set the right class reference in the configuration file.