cookie丢失
[NullReferenceException: 未将对象引用设置到对象的实例。]

[解决办法]
HttpCookie MyCookie = new HttpCookie("LastVisit");

DateTime now = DateTime.Now;

MyCookie.Value = now.ToString();

MyCookie.Expires = now.AddHours(1);
Response.Cookies.Add(MyCookie);

要设置cookie的过期时间，不然下次浏览器关闭后自动过期