public static string GetAccessToken()
        {
            string _AccessToken = "";//存储微信访问凭证
            DateTime _lastGetTimeOfAccessToken;//ACCESS_TOKEN最后一次更新时间

            string corpId = "XXXXX";// 填入企业的corpId
            string corpsecret = "XXXX";// 填入Secret

            try
            {
                string sqlstring = "SELECT count(*)  FROM [Booking].[dbo].[WX_GetAccessCredentials] "
                    + " WHERE WX_LastGetTimeOfAccessToken> GETDATE(); ";
                DataTable dt = DBUtility.DbHelperSQL.Query(sqlstring);

                if (dt.Rows[0][0].ToString() == "0") //_lastGetTimeOfAccessToken < DateTime.Now
                {
                    string serviceAddress = string.Format("https://qyapi.weixin.qq.com/cgi-bin/gettoken?corpid={0}&corpsecret={1}", corpId, corpsecret);
                    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(serviceAddress);
                    request.Method = "GET";
                    request.ContentType = "text/html;charset=UTF-8";
                    HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                    Stream myResponseStream = response.GetResponseStream();
                    StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.UTF8);
                    string retString = myStreamReader.ReadToEnd();
                    myStreamReader.Close();
                    myResponseStream.Close();

                    var rsEntity = new { access_token = "", expires_in = 0, errcode = 0, errmsg = "" };
                    dynamic en = Newtonsoft.Json.JsonConvert.DeserializeAnonymousType<object>(retString, rsEntity); // Newtonsoft.Json提供的匿名类反序列化
                    _lastGetTimeOfAccessToken = DateTime.Now.AddSeconds((double)en.expires_in - 300);
                    _AccessToken = en.access_token;//GetAccessToken

                    string sqlstring1 = "insert into [Booking].[dbo].[WX_GetAccessCredentials] ([WX_LastGetTimeOfAccessToken],[WX_AccessToken],[Create_Date])"
                        + "VALUES('" + _lastGetTimeOfAccessToken + "','" + _AccessToken + "',GETDATE())";

                    var result = DBUtility.DbHelperSQL.ExecuteSql(sqlstring1);
                }

                string sqlstring3 = "SELECT [WX_LastGetTimeOfAccessToken],[WX_AccessToken],[Create_Date]  FROM [Booking].[dbo].[WX_GetAccessCredentials] "
                    + " WHERE WX_LastGetTimeOfAccessToken> GETDATE(); ";
                DataTable dt3 = DBUtility.DbHelperSQL.Query(sqlstring3);
                _AccessToken = dt3.Rows[0][1].ToString();

                return _AccessToken;
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

            //获取图片路径
            var t1 = retString.IndexOf("http://");
            var t2 = retString.IndexOf("\",\"status\"");
            retString = retString.Substring(t1, t2 - t1);

            //下载图片
            string newname = DateTime.Now.ToString("yyyyMMddHHmmssfff") + ".jpg";
            string datePath = System.Web.HttpContext.Current.Server.MapPath("/") + "Upload/Test/" + newname;
            using (System.Net.WebClient wc = new System.Net.WebClient())
            {
                wc.Headers.Add("User-Agent", "Chrome");
                wc.DownloadFile(retString, @"" + datePath + "");
                //wc.DownloadFile(retString, @"d:\mobile.gif");//保存到本地的文件名和路径，请自行更改
            }

            //图片处理
            string PhotoName = "1.jpg";
            string filepath= System.Web.HttpContext.Current.Server.MapPath("/") + "Upload/Test/" + PhotoName;
            MediaType mt = MediaType.image;
            string media_id = WxUpLoad(filepath, mt);

//下面是获取media_id的方法
        /// <summary>
        /// topic：用WebClient类来上传文件，并读出返回值media_id
        /// </summary>
        /// <param name="filepath">文件绝对路径</param>
        /// <param name="mt">文件类型</param>
        public string WxUpLoad(string filepath, MediaType mt)
        {
            string media_id = "";
            string token = GetAccessToken();
            using (WebClient client = new WebClient())
            {
                byte[] b = client.UploadFile(string.Format("https://qyapi.weixin.qq.com/cgi-bin/media/upload?access_token={0}&type={1}", token, mt.ToString()), filepath);//调用接口上传文件
                string retdata = Encoding.Default.GetString(b);//获取返回值
                if (retdata.Contains("media_id"))//判断返回值是否包含media_id，包含则说明上传成功，然后将返回的json字符串转换成json
                {
                    media_id=JsonConvert.DeserializeObject<UpLoadInfo>(retdata).media_id;
                    return media_id;
                }
                else
                {return null;}
            }
        }
//里面有枚举
        /// <summary>
        /// topic：媒体文件的类型定义为枚举
        /// </summary>
        public enum MediaType
        {
            /// <summary>
            /// 图片（image）：2MB，支持JPG,PNG格式
            /// </summary>
            image,

            /// <summary>
            /// 语音（voice） ：2MB，播放长度不超过60s，仅支持AMR格式
            /// </summary>
            voice,

            /// <summary>
            /// 视频（video）：10MB，支持MP4格式
            /// </summary>
            video,

            /// <summary>
            /// 普通文件（file）：20MB
            /// </summary>
            file
        }

//1.在文件顶部引用命名空间
//2.为命名空间或类型定义别名
//3.使用using，定义范围，在该范围结束时回收资源