/// <summary>
        /// 获取凭证接口
        /// </summary>
        /// <param name="grant_type">获取access_token填写client_credential</param>
        /// <param name="appid">第三方用户唯一凭证</param>
        /// <param name="secret">第三方用户唯一凭证密钥，既appsecret</param>
        /// <returns></returns>
        public static string GetAccessToken(string appid, string secret)
        {
            string token = OperationXml.GetXMLToken();
            if (token == "")
            {
                string strJson = RequestUrl(string.Format("https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid={0}&secret={1}", appid, secret));
                token = GetJsonValue(strJson, "access_token");
                OperationXml.UpdateXMLToken(token, DateTime.Now);
            }

            return token;
        }
 public static string RequestUrl(string strUrl)
        {
            // 设置参数
            HttpWebRequest request = WebRequest.Create(strUrl) as HttpWebRequest;
            CookieContainer cookieContainer = new CookieContainer();
            request.CookieContainer = cookieContainer;
            request.AllowAutoRedirect = true;
            request.Method = "POST";
            request.ContentType = "text/html";
            request.Headers.Add("charset", "utf-8");

            //发送请求并获取相应回应数据
            HttpWebResponse response = request.GetResponse() as HttpWebResponse;
            //直到request.GetResponse()程序才开始向目标网页发送Post请求
            Stream responseStream = response.GetResponseStream();
            StreamReader sr = new StreamReader(responseStream, Encoding.UTF8);
            //返回结果网页（html）代码
            string content = sr.ReadToEnd();
            return content;
        }

#region 获取Json字符串某节点的值

        /// <summary>
        /// 获取Json字符串某节点的值
        /// </summary>
        public static string GetJsonValue(string jsonStr, string key)
        {
            string result = string.Empty;
            if (!string.IsNullOrEmpty(jsonStr))
            {
                key = "\"" + key.Trim(‘"‘) + "\"";
                int index = jsonStr.IndexOf(key) + key.Length + 1;
                if (index > key.Length + 1)
                {
                    //先截逗号，若是最后一个，截“｝”号，取最小值
                    int end = jsonStr.IndexOf(‘,‘, index);
                    if (end == -1)
                    {
                        end = jsonStr.IndexOf(‘}‘, index);
                    }

                    result = jsonStr.Substring(index, end - index);
                    result = result.Trim(new char[] { ‘"‘, ‘ ‘, ‘\‘‘ }); //过滤引号或空格
                }
            }
            return result;
        }

        #endregion

  /// <summary>
        /// 获取XML文件路径
        /// </summary>
        public static string xmlName = AppDomain.CurrentDomain.BaseDirectory + @"\AccessToken.xml";

        /// <summary>
        /// 读取XML  Token
        /// </summary>
        /// <param name="xmlUrl"></param>
        /// <returns></returns>
        public static string GetXMLToken()
        {
            try
            {
                XmlDocument xmlDoc = new XmlDocument();
                xmlDoc.Load(xmlName);

                XmlNode xn = xmlDoc.SelectSingleNode("ACCESS_TOKEN");
                XmlElement xe = (XmlElement)xn;//将子节点类型转换为XmlElement类型 
                string Token = "";
                string CreateTime = "";
                foreach (XmlNode xn1 in xe)//遍历 
                {
                    XmlElement xe2 = (XmlElement)xn1;//转换类型 
                    if (xe2.Name == "Token")//如果找到 
                    {
                        Token = xe2.InnerText;//则修改
                    }
                    if (xe2.Name == "CreateTime")//如果找到 
                    {
                        CreateTime = xe2.InnerText;//则修改
                    }
                }

                int seconds = 0;

                if (CreateTime.Trim() != "")
                {
                    DateTime ct = Convert.ToDateTime(CreateTime);
                    TimeSpan ts = (DateTime.Now - ct);
                    seconds = ts.Milliseconds;
                }
                LogTextHelper.WriteLogToFile("Token：" + Token);
                if (seconds > 7200 || seconds == 0)
                {
                    return "";
                }
                else
                {
                    return Token;
                }
            }
            catch (Exception ex)
            {
                LogTextHelper.WriteLogToFile("异常：" + ex.ToString());
                return "";
            }
        }

        /// <summary>
        /// 修改Token
        /// </summary>
        /// <param name="token"></param>
        /// <param name="createTime"></param>
        public static void UpdateXMLToken(string token, DateTime createTime)
        {
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load(xmlName);

            XmlNode xn = xmlDoc.SelectSingleNode("ACCESS_TOKEN");
            XmlElement xe = (XmlElement)xn;//将子节点类型转换为XmlElement类型 

            foreach (XmlNode xn1 in xe)//遍历 
            {
                XmlElement xe2 = (XmlElement)xn1;//转换类型 
                if (xe2.Name == "Token")//如果找到 
                {
                    xe2.InnerText = token;//则修改
                }
                if (xe2.Name == "CreateTime")//如果找到 
                {
                    xe2.InnerText = createTime.ToString();//则修改
                }
            }
            xmlDoc.Save(xmlName);

        }