Null username

System.Security.Principal.WindowsPrincipal p = System.Threading.Thread.CurrentPrincipal as System.Security.Principal.WindowsPrincipal;
string strUsername = p.Identity.Name;
 
I am using the above code to get the username. This works fine in the development environment but when I copy all my files to the virtual directory and run it using the browser, the username is not obtained.
Can anyone help? ...