清除cookies

            HttpCookie aCookie;
            string cookieName;
            int limit = Request.Cookies.Count;
            for (int i = 0; i < limit; i++)
            {
                cookieName = Request.Cookies[i].Name;
                aCookie = new HttpCookie(cookieName);
                aCookie.Expires = DateTime.Now.AddDays(-1);
                Response.Cookies.Add(aCookie);
            }