¿Por qué User.Identity.IsAuthenticated siempre es falso?

Hola a todos,
Quiero obtener el nombre de usuario a continuación

<pre lang="c#">

 public ActionResult Login(LoginModel model, string returnUrl)
        {
 if (User.Identity.IsAuthenticated)//this is get always false
            {
string user = User.Identity.Name;//here i need username
            }
  
            if (ModelState.IsValid && WebSecurity.Login(model.UserName, model.Password,     persistCookie: model.RememberMe))
            {
                return RedirectToLocal(returnUrl);
            }

            // If we got this far, something failed, redisplay form
            ModelState.AddModelError("", "The user name or password provided is incorrect.");
            return View(model);
        }

User.Identity.IsAuthenticated always return false so userid comes always -1, I am not even able to short out my mistake, what mistake am doing and how can i overcome with this, please help me..