HttpContext.Current.Server.MapPath("") 未将对象设置到引用的

在多线程中使用该方法获取目录报错：未将对象设置到引用

 #region 上传图片到腾讯
         public async Task<Result> UploadImageToWX(string background_pic_url)
         {
             var _result = new Result();
             var token = GetWxGhAuthorizerAccessToken(AbpSession.ShopId);
             if (!token.success)
             {
                 _result.Code = ;
                 _result.Message = token.message;
                 return _result;
             }

             try
             {
                 var filename = background_pic_url.Substring(background_pic_url.LastIndexOf('/'));
                 if (filename.LastIndexOf('.') < )
                 {
                     _result.Code = ;
                     _result.Message = "图片错误";
                     return _result;
                 }
                 var suffix = filename.Substring(filename.LastIndexOf('.'));

                 //报错代码
                 //string path = System.Web.HttpContext.Current.Server.MapPath("/upload/images/" + DateTime.Now.ToString("yyyyMM"));

                 //解决方案
                 string path = MapPath("/upload/images/" + DateTime.Now.ToString("yyyyMM"));

                 if (!Directory.Exists(path))
                 {
                     Directory.CreateDirectory(path);
                 }
                 string file = path + "/" + Guid.NewGuid().ToString("n") + suffix;

                 Uri uri = new Uri(background_pic_url);
                 HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(uri);
                 HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                 Stream resStream = response.GetResponseStream();//得到图片数据流
                 Bitmap sourcebm = new Bitmap(resStream);//初始化Bitmap图片
                 sourcebm.Save(file);

                 var tokenResult = token.data as token_model;

                 var uploadResult = await Senparc.Weixin.MP.AdvancedAPIs.MediaApi.UploadImgAsync(tokenResult.token, file);
                 _result.Code = ;
                 _result.Data = uploadResult;
             }
             catch (Exception ex)
             {

                 _result.Message = ex.Message;

                 _result.Code = ;

             }

             return _result;
         }
         #endregion

解决方法：

     /// <summary>
         /// 获取文件路径
         /// </summary>
         /// <param name="strPath"></param>
         /// <returns></returns>
         private string MapPath(string strPath)
         {
             if (HttpContext.Current != null)
             {
                 return HttpContext.Current.Server.MapPath(strPath);
             }
             else //非web程序引用             
             {
                 strPath = strPath.Replace("/", "\\");
                 if (strPath.StartsWith("\\"))
                 {
                     strPath = strPath.TrimStart('\\');
                 }
                 return System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, strPath);
             }
         }