使用者身份GetUserId()在构造函数中返回null

使用者身份GetUserId()在控制器构造函数中返回null 这是我的表格:

@using (Html.BeginForm("Login", "Account", new { ReturnUrl = ViewBag.ReturnUrl }, FormMethod.Post, new { @class = "form-horizontal", role = "form" }))
{
    @Html.ValidationSummary("", new { @class = "text-danger" })
    @Html.AntiForgeryToken()

    @Html.TextBoxFor(m => m.Email, new { @class = "form-control", placeholder = "Email" })         
    @Html.PasswordFor(m => m.Password, new { @class = "form-control", placeholder = "Mot de passe" })

    <input type="submit" id="ModalSubmitButton" class="btn btn-primary btn-lg btn-block login-button" value="Se connecter" />
}

然后在account Controller中的登录操作中:

[HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Login(LoginViewModel model, string returnUrl)
        {
    var result = await SignInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, shouldLockout: false);
                switch (result)
                {
                    case SignInStatus.Success:
                        //return RedirectToLocal(returnUrl);
                        return RedirectToAction("Index", "User");
                    case SignInStatus.LockedOut:
                        return View("Lockout");
                    case SignInStatus.RequiresVerification:
                        return RedirectToAction("SendCode", new { ReturnUrl = returnUrl, RememberMe = model.RememberMe });
                    case SignInStatus.Failure:
                    default:
                        ModelState.AddModelError("", "Tentative de connexion non valide.");
                        return PartialView("_SignIn",model);
                }

在userController构造函数中,我有:

public ActionResult Index()
        {
            var x = User.Identity.GetUserId();
            return View();
        }

更新:我使用Int作为ID,而不是string