Implement Cookie in Sahrepoint

Hi,

I have requirement of implementin cookie in my web part.I have used the following code.

HttpCookie objPollingCookie = HttpContext.Current.Response.Cookies[strListURL + "|" + iListItemID + "|" + SPContext.Current.Web.CurrentUser.ID];

            if ((objPollingCookie != null) && (objPollingCookie.Value != null))
            {
                //HttpCookie objPollingCookie = this.Page.Response.Cookies[strListURL + "|" + iListItemID + "|" + SPContext.Current.Web.CurrentUser.ID];

                Utils objDecrypt=new Utils();
                string strVoteStatus = objDecrypt.DecryptString(objPollingCookie.Value);
                if (strVoteStatus == "Yes")
                {
                    return true;
                }
                else
                {
                   return  false;
                }
}

The cookie never returns false cookie is present their with some default values with date as 0001-01-01 ,ie why I have addded the conditon for cookie.value==null. The cookie I am writing are also not geting saved .Is there anything I am missing for implementing the cookie in sharepoint.COde for writing cookie is given below

HttpCookie objPollingCookie = new HttpCookie(strListURL+"|"+iListItemID+"|"+SPContext.Current.Web.CurrentUser.ID);

            objPollingCookie.Value = objEncrypt.EncryptData("Yes");
            objPollingCookie.Expires = DateTime.Now.AddMonths(1);
            HttpContext.Current.Response.Cookies.Add(objPollingCookie);