ServiceHelper

     public class ServiceHelper
     {
         private static string _baseUrl = $@"http://{Config.Instance.MesServerUrl}/";
         private static string _sessionHeader;
         public static void ClearSession()
         {
             _sessionHeader = "";
         }
         public static string SessionId;

         public static string BaseUrl
         {
             get { return _baseUrl; }
             set { _baseUrl = $@"http://{value}/";} //value  包含IP和端口号 + 后台服务名， 如:localhost:8080/DadeMes
         } 

         /// <summary>
         /// HttpClient实现Get请求
         /// <param name="subUrl">子URL前面必须带 "/" 符号</param>
         /// </summary>
         public static string Get(string subUrl)
         {
             try
             {
                 using (var client = GetClient())
                 {
                     return client.DownloadString(subUrl);

                 }
             }
             catch (WebException)
             {
                 throw new Exception("无法连接服务器");
             }
         }

         /// <summary>
         /// Post
         /// </summary>
         /// <param name="url">子地址</param>
         /// <param name="strData">要传输的json字符串</param>
         /// <returns></returns>
         public static string Post(string url, string strData = "")
         {
             try
             {
                 using (var client = GetClient())
                 {
                     var postData = Encoding.UTF8.GetBytes(strData); //编码，尤其是汉字，事先要看下抓取网页的编码方式
                     var responseData = client.UploadData(url, "POST", postData); //得到返回字符流
                     if (string.IsNullOrWhiteSpace(_sessionHeader))
                     {
                         _sessionHeader = client.ResponseHeaders["Set-Cookie"];//保存session
                     }
                     return Encoding.UTF8.GetString(responseData); //解码
                 }
             }
             catch (WebException ex)
             {
                 throw new Exception("无法连接服务器");
             }
         }

         /// <summary>
         /// 将本地文件上传到服务器
         /// </summary>
         /// <param name="url">服务器路径</param>
         /// <param name="filePath">本地路径</param>
         /// <returns>上传成功后返回的是上文件的路径</returns>
         public static string UploadFile(string url, string filePath)
         {
             try
             {
                 using (var client = GetClient())
                 {
                     var responseData = client.UploadFile(url, filePath); //得到返回字符流

                     if (string.IsNullOrWhiteSpace(_sessionHeader))
                     {
                         _sessionHeader = client.ResponseHeaders["Set-Cookie"];//保存session
                     }
                     return Encoding.UTF8.GetString(responseData); //解码
                 }
             }
             catch (WebException ex)
             {
                 throw new Exception("无法连接服务器");
             }
         }

         private static WebClient GetClient()
         {
             return new WebClient
             {
                 //webClient.Headers.Add("Cookie",header);//在给服务器发送消息时加上这个Session值
                 Headers =
                 {
                     [HttpRequestHeader.UserAgent] = "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)",
                     ["DNT"] = "",
                     //["Content-Type"]="application/x-www-form-urlencoded",
                     ["Content-Type"]="application/json",
                     ["Cookie"]=_sessionHeader,
                     ["x-requested-with"]="XMLHttpRequest",
                 },
                 Encoding = Encoding.UTF8,
                 BaseAddress = BaseUrl
             };
         }
     }