HttpContext.Current.User.Identity.Name Problem (Urgent)

My problem is HttpContext.Current.User.Identity.Name is always empty the first time i run it , so when i run it again two things haapens

for example in my code I have this

FormsAuthentication.SetAuthCookie("mik", true);

now the value of name  will be mik (this is i run it the second time)

string name = HttpContext.Current.User.Identity.Name;

 

 now if i run it the third time and i pass

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