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);

             }

         }