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

            // 生成令牌并发送该令牌
            if (!await SignInManager.SendTwoFactorCodeAsync(model.SelectedProvider))
            {
                return(View("Error"));
            }
            return(RedirectToAction("VerifyCode", new { Provider = model.SelectedProvider, ReturnUrl = model.ReturnUrl, RememberMe = model.RememberMe }));
        }