C#微信公众号开发——access_token的获取

access_token是公众号的全局唯一票据，公众号调用各接口时都需使用access_token。正常情况下access_token有效期为7200秒，重复获取将导致上次获取的access_token失效。

公众号可以使用AppID和AppSecret调用本接口来获取access_token。AppID和AppSecret可在微信公众平台官网-开发者中心页中获得（需要已经成为开发者，且帐号没有异常状态）。注意调用所有微信接口时均需使用https协议。

一、获取access_token

 /// <summary>
        /// 通过appID和appsecret获取Access_token
        /// </summary>
        /// <returns></returns>
        private static Access_token GetAccess_token()
        {
            string appid = "appid";
            string secret = "appsecret";
            string strUrl = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=" + appid + "&secret=" + secret;
            Access_token mode = new Access_token();
            HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(strUrl);
            req.Method = "GET";
            using (WebResponse wr = req.GetResponse())
            {
                HttpWebResponse myResponse = (HttpWebResponse)req.GetResponse();
                StreamReader reader = new StreamReader(myResponse.GetResponseStream(), Encoding.UTF8);
                string content = reader.ReadToEnd();//在这里对Access_token 赋值
                Access_token token = new Access_token();
                token = JsonHelper.ParseFromJson<Access_token>(content);
                mode.access_token = token.access_token;
                mode.expires_in = token.expires_in;
            }
            return mode;
        }

二、由于access_token会过期，所以获取access_token的时候要验证是否过期了

  /// <summary>
        /// 获取Access_token值
        /// </summary>
        /// <returns></returns>
        public static string IsExistAccess_Token()
        {
            string Token = string.Empty;
            DateTime YouXRQ;
            // 读取XML文件中的数据，并显示出来 ，注意文件路径
            string filepath = System.Web.HttpContext.Current.Server.MapPath("XMLFile.xml");
            StreamReader str = new StreamReader(filepath, System.Text.Encoding.UTF8);
            XmlDocument xml = new XmlDocument();
            xml.Load(str);
            str.Close();
            str.Dispose();
            Token = xml.SelectSingleNode("xml").SelectSingleNode("Access_Token").InnerText;
            YouXRQ = Convert.ToDateTime(xml.SelectSingleNode("xml").SelectSingleNode("Access_YouXRQ").InnerText);
            if (DateTime.Now > YouXRQ)
            {
                DateTime _youxrq = DateTime.Now;
                Access_token mode = GetAccess_token();
                xml.SelectSingleNode("xml").SelectSingleNode("Access_Token").InnerText = mode.access_token;
                _youxrq = _youxrq.AddSeconds(int.Parse(mode.expires_in));
                xml.SelectSingleNode("xml").SelectSingleNode("Access_YouXRQ").InnerText = _youxrq.ToString();
                xml.Save(filepath);
                Token = mode.access_token;
            }
            return Token;
        }

三、