I have an asp.net web forms application running under v4.0 integrated mode.
I tried to add an apicontroller in the App_Code folder.
In the Global.asax, I ad
Also try to delete your entire api bin folder's content. Mine was containing old dlls (due to a big namespace renaming) exposing conflicting controllers. Those dll weren't deleted by Visual Studio's Clean functionality.
(However, I find asp.net web api seriously lacks routing and debugging information at the debugging level).