public async Task<ActionResult> VerifyCode(VerifyCodeViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return View(model);
            }

            // 以下代码可以防范双重身份验证代码遭到暴力破解攻击。
            // 如果用户输入错误代码的次数达到指定的次数,则会将
            // 该用户帐户锁定指定的时间。
            // 可以在 IdentityConfig 中配置帐户锁定设置
            var result = await SignInManager.TwoFactorSignInAsync(model.Provider, model.Code, isPersistent: model.RememberMe, rememberBrowser: model.RememberBrowser);
            switch (result)
            {
                case SignInStatus.Success:
                    return RedirectToLocal(model.ReturnUrl);

                case SignInStatus.LockedOut:
                    return View("Lockout");

                case SignInStatus.Failure:
                default:
                    ModelState.AddModelError("", "代码无效。");
                    return View(model);
            }
        }
        public async Task<ActionResult> VerifyCode(VerifyCodeViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return View(model);
            }

            // The following code protects for brute force attacks against the two factor codes. 
            // If a user enters incorrect codes for a specified amount of time then the user account 
            // will be locked out for a specified amount of time. 
            // You can configure the account lockout settings in IdentityConfig
            var result = await SignInManager.TwoFactorSignInAsync(model.Provider, model.Code, isPersistent:  model.RememberMe, rememberBrowser: model.RememberBrowser);
            switch (result)
            {
                case SignInStatus.Success:
                    return RedirectToLocal(model.ReturnUrl);
                case SignInStatus.LockedOut:
                    return View("Lockout");
                case SignInStatus.Failure:
                default:
                    ModelState.AddModelError("", "Invalid code.");
                    return View(model);
            }
        }