You can store source code in the App_Code folder, and it will be automatically compiled at run time. The resulting assembly is accessible to any other code in the Web application. The App_Code folder therefore works much like the Bin folder, except that you can store source code in it instead of compiled code. The App_Code folder and its special status in an ASP.NET Web application makes it possible to create custom classes and other source-code-only files and use them in your Web application without having to compile them independently.