//获取微信凭证access_token的接口
        public static string getAccessTokenUrl = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid={0}&secret={1}";

        #region 获取微信凭证
        public string GetAccessToken(string wechat_id)
        {
            string accessToken = "";
            //获取配置信息Datatable
            DataTable dtwecaht = wechatdal.GetList("wechat_id=‘" + wechat_id + "‘").Tables[0];

            if (dtwecaht.Rows.Count > 0)
            {
                string respText = "";
                //获取appid和appsercret
                string wechat_appid = dtwecaht.Rows[0]["wechat_appid"].ToString();
                string wechat_appsecret = dtwecaht.Rows[0]["wechat_appsecret"].ToString();
                //获取josn数据
                string url = string.Format(getAccessTokenUrl, wechat_appid, wechat_appsecret);

                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();

                using (Stream resStream = response.GetResponseStream())
                {
                    StreamReader reader = new StreamReader(resStream, Encoding.Default);
                    respText = reader.ReadToEnd();
                    resStream.Close();
                }
                JavaScriptSerializer Jss = new JavaScriptSerializer();
                Dictionary<string, object> respDic = (Dictionary<string, object>)Jss.DeserializeObject(respText);
                //通过键access_token获取值
                accessToken = respDic["access_token"].ToString();
            }

            return accessToken;
        }
        #endregion 获取微信凭证

.Net实现微信公众平台开发接口（二） 之 “获取access_token”