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 = 0;
                _result.Message = token.message;
                return _result;
            }

            try
            {
                var filename = background_pic_url.Substring(background_pic_url.LastIndexOf('/'));
                if (filename.LastIndexOf('.') < 0)
                {
                    _result.Code = 0;
                    _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 = 1;
                _result.Data = uploadResult;
            }
            catch (Exception ex)
            {

                _result.Message = ex.Message;

                _result.Code = 0;

            }


            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);
            }
        }