C# v3微信 access token 过期处理的问题

        //记录access token 申请时的时间
        private static DateTime GetAccessToken_Time;  

        /// <summary>
        /// 过期时间为7200秒
        /// </summary>
        private static int Expires_Period = 7200;

 　　　　　　private static string mAccessToken;

        /// <summary>
        /// 获取access token
        /// </summary>
        /// <returns></returns>
        private static string GetAccessToken()
        {
            if (string.IsNullOrEmpty(mAccessToken) || HasExpired())
            {
                string Url = "https://api.weixin.qq.com/cgi-bin/token";
                string RequestPara = "grant_type=client_credential&appid=" + APPID + "&secret=" + APPSECRET;
                string result = WebRequestHelper.GetData(RequestPara, Url);
                if (!result.Equals(""))
                {
                    GetAccessToken_Time = DateTime.Now;
                    accesstoken token = JsonFormat.Deserialize<accesstoken>(result);
                    Expires_Period = Convert.ToInt32(token.expires_in);
                    return token.access_token;
                }
                else
                {
                    GetAccessToken_Time = DateTime.MinValue;
                }
            }
            return null;
        }

        /// <summary>
        /// 判断Access_token是否过期
        /// </summary>
        /// <returns>bool</returns>
        private static bool HasExpired()
        {
            if (GetAccessToken_Time != null)
            {
                //过期时间，允许有一定的误差，一分钟。获取时间消耗
                if (DateTime.Now > GetAccessToken_Time.AddSeconds(Expires_Period).AddSeconds(-60))
                {
                    return true;
                }
            }
            return false;
        }