Windows Authentication Issue

Hi

Windows Identity becoming null after browsing the application for some time. Below is the code i am using to get the Windows User.

We are using .NET Framework 3.5 and IIS 7.0 & 7.5.

IIdentity WinId = HttpContext.Current.User.Identity;
WindowsIdentity wi = WinId as WindowsIdentity;

i am trying to get the windows name using wi.Name.

Thanks for the support and time.

Best Regards,
Venkatesh

Thanks & Regards
VENKI