Chack user authenication in IIS issue

Hi,
I want to check if user is autheneicated in control (ascx). I use HttpContext.Current.User.Identity.IsAuthenticated; for this purpose.
When I browse pages using ASP.Net Developement server -VS 2008- it works fine, But when I use IIS 7 then it always return false , even if user is authenicated.
Note that it's even work fine with IIS 6 but With IIS no hope (Classic/Integrated mode).
How could I solve it. does it related to IIS 7 or what the method that I use?
 
Thanks

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