public ActionResult LogOn(string myCaptcha,string attempt,User userToLogin,string returnUrl)
        {
            if (!CaptchaHelper.VerifyAndExpireSolution(HttpContext, myCaptcha, attempt))
            {
                // Redisply the view width an error message
                ModelState.AddModelError("", "验证码错误！");
            }

            if (ModelState.IsValid)
            {
                var users = unitOfWork.UsersRepository.Get(
                filter: x => x.UserName == userToLogin.UserName && x.Password == userToLogin.Password
                    );

                if (users.Count() > 0) // login succeed
                {
                    Session["UserID"] = users.First();

                    return Redirect(returnUrl??"~/Admin");
                }
                else
                {
                    ModelState.AddModelError(string.Empty, "登入失败，用户名或密码错误");
                }
            }

            return View();
        }