>>>
>>>Which directory should I add .cs files in order to be able to use them in the project? In App_Code ?
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>>If you are using a "Web Application"-style project, it doesn't really matter. Pick a place where it makes the most sense to you. A webapp project compiles your DLL's using normal filenames (eg. they're usually the project name assuming you didn't override this). If it's a website project, yeah, drop the class in App_Code.
>
>I'm not sure which type it is. Anyway, I dropped them in App_Code, I'm having this problem now

Did you happen to add a reference to the existing project already? If so, remove it. It's either an eternal/referenced DLL or it's part of the same project. You don't want to do both.