>>Hi everybody,
>>
>>I've added a class (.cs) to the App_Code. I tried to build solution multiple times, but the dll for this class is not created (I deleted the dll from the bin folder with other files).
>
>An assembly (DLL) is only built per-project, not per class.
>
>I really haven't used App_Code or that model for web development so I don't have any insight into it.

The App_Code directory contains files you want compiled by the web server. Unless you pre-complile everything in advance, the web server will always compare what is in the App_Code folder and re-compile it when it is newer. If you are using a Web Application Project model then the App_Code folder if present will give you a build error. I won't create any more web apps without using the project model.
Tim