Cookie应用一个月只能投一次票

//进行投票
protected void Button1_Click(object sender, EventArgs e)
{
//判断指定的IP是否已在本月投过票了,如果已经投过了,则弹出提示对话框
string UserIP = Request.UserHostAddress.ToString( );
int VoteID = Convert.ToInt32(RadioButtonList1.SelectedIndex.ToString( ))+1;
HttpCookie oldCookie=Request.Cookies["userIP"];
if (oldCookie == null)
{
UpdateVote(VoteID);
Response.Write("<script>alert('投票成功，谢谢您的参与！')</script>");
HttpCookie newCookie = new HttpCookie("userIP");       //定义新的Cookie对象
newCookie.Expires = DateTime.Now.AddMonths(1);
//添加新的Cookie变量IPaddress，值为UserIP
newCookie.Values.Add("IPaddress", UserIP);
Response.AppendCookie(newCookie);            //将变量写入Cookie文件中
return;
}
else
{
string userIP = oldCookie.Values["IPaddress"];
if (UserIP.Trim( ) == userIP.Trim( ))
{
Response.Write("<script>alert('一个IP地址
一个月内只能投一次票，谢谢您的参与！');history.go(-1);获至宝</script>");
return;
}
else
{
HttpCookie newCookie = new HttpCookie("userIP");