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>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.