Nuget安装RestSharp版本，不同版本之间的区别有很大，当前这个写法基于以下版本

  public class APIHelper
  {
      private readonly string baseUrl = ConfigurationManager.AppSettings["connectionString"].ToString(); 

      /// <summary>
      /// http请求接口
      /// </summary>
      /// <param name="url">地址</param>
      /// <param name="data">数据，json格式化后的数据</param>
      /// <param name="method">方法 POST，或者get</param>
      /// <returns></returns>
      public string HttpRequest(string url, string data, Method method, string file = null)
      {
          try
          {
              var client = new RestClient(baseUrl + url);
              var request = new RestRequest(method);
              request.RequestFormat = DataFormat.Json;
              if (file == null)
                  request.AddHeader("Content-Type", "application/json");
              else
              {
                  request.AddHeader("Content-Type", "multipart/form-data");
                  request.AddHeader("user_id", UserCache.Current_UserId.ToString());
                  request.AddHeader("username", System.Web.HttpUtility.UrlEncode(UserCache.Current_UserName));
              }
              request.Timeout = 10000;
              if (data != null)
                  request.AddParameter("application/json; charset=utf-8", data, ParameterType.RequestBody);
              if (UserCache.CACHE_UserInfoModel != null)
                  request.AddHeader("authorization", "Bearer " + UserCache.CACHE_UserInfoModel.access_token);
              if (file != null)
                  request.AddFile("file", file);
              IRestResponse response = client.Execute(request);
              var responseContent = response.Content;
              return responseContent;
          }
          catch (Exception ex)
          {
              return null;
          }
      }
  }

接口调用