.NET 添加/删除Cookie

public class CookieHelper
    {
        /// <summary>
        /// 清除指定Cookie
        /// </summary>
        /// <param name="cookiename">cookiename</param>
        public static void ClearCookie(string cookiename)
        {
            HttpCookie cookie = HttpContext.Current.Request.Cookies[cookiename];
            if (cookie != null)
            {
                cookie.Expires = DateTime.Now.AddYears(-);
                HttpContext.Current.Response.Cookies.Add(cookie);
            }
        }
        /// <summary>
        /// 获取指定Cookie值
        /// </summary>
        /// <param name="cookiename">cookiename</param>
        /// <returns></returns>
        public static string GetCookieValue(string cookiename)
        {
            HttpCookie cookie = HttpContext.Current.Request.Cookies[cookiename];
            string str = string.Empty;
            if (cookie != null)
            {
                str = cookie.Value;
            }
            return str;
        }
        /// <summary>
        /// 添加一个Cookie（24小时过期）
        /// </summary>
        /// <param name="cookiename"></param>
        /// <param name="cookievalue"></param>
        public static void SetCookie(string cookiename, string cookievalue)
        {
            SetCookie(cookiename, cookievalue, DateTime.Now.AddDays(1.0));
        }
        /// <summary>
        /// 添加一个Cookie
        /// </summary>
        /// <param name="cookiename">cookie名</param>
        /// <param name="cookievalue">cookie值</param>
        /// <param name="expires">过期时间 DateTime</param>
        public static void SetCookie(string cookiename, string cookievalue, DateTime expires)
        {
            HttpCookie cookie = new HttpCookie(cookiename)
            {
                Value = cookievalue,
                Expires = expires
            };
            HttpContext.Current.Response.Cookies.Add(cookie);
        }
    }