Parallel.ForEach错误HttpContext.Current

这种方法 - doDayBegin(item.BranchId)需要很长时间才能执行.所以我用Parallel.ForEach它来并行执行它.当我使用正常foreach循环时它的工作正常但当我使用Parallel.ForEach它时显示此错误
对象引用未设置为对象的实例.

 public ActionResult Edit([DataSourceRequest] DataSourceRequest request)
        {
            try
            {
                JavaScriptSerializer js = new JavaScriptSerializer();
                List _listDB0010020Vm = new List();

                string dataDB0010020vm = Request.Form["griddetailsvm"];
                if (!string.IsNullOrEmpty(dataDB0010020vm))
                {
                    _listDB0010020Vm = js.Deserialize>(dataDB0010020vm).
                    Where(d => d.IsValid == "YES").ToList();
                }
                DateTime start = DateTime.UtcNow;


                Parallel.ForEach(_listDB0010020Vm, item =>
                {
                    doDayBegin(item.BranchId);
                });

                DateTime end = DateTime.UtcNow;
                TimeSpan duration = end - start;
                return Json(new
                {
                    success = true,
                    message = "Day Begin Process Completed Successfully!" + duration
                });
            }
            catch (Exception e)
            {
                return Json(new
                {
                    success = false,
                    message = e.Message
                });

            }
        }

  public void doDayBegin(int BranchId)
{
    var httpContext = System.Web.HttpContext.Current;
    IDB0010020Repository _idDB0010020Repository = new DB0010020Repository();
    IDB0010044Repository _idDB0010044Repository = new DB0010044Repository();

     EBS.DAL.Model.DB0010020 branchDetails = _idDB0010020Repository.FindOne(d => d.BranchId == BranchId);
    if (branchDetails == null)
    {
        ModelState.AddModelError("", "Branch not found!");
    }
    else
    {
        try
        {
            DateTime LastOpenDate = DateTime.ParseExact(Request.Form["LastOpenDate"].ToString(), "dd/MM/yyyy", CultureInfo.InvariantCulture);
         //   branchDetails.LastOpenDate = LastOpenDate;
    //      branchDetails.LastOpenDate = Convert.ToDateTime(Request.Form["LastOpenDate"].ToString());


        }
        catch (Exception e)
        {
          //  branchDetails.LastOpenDate = Convert.ToDateTime("2014-07-25 00:00:00.000");
        }


        OperationStatus status = _idDB0010020Repository.UpdateAndSave(branchDetails);
        if (status != null && !status.Status)
            ModelState.AddModelError("Updation failed", status.ExceptionMessage);
    }

    EBS.DAL.Model.DB0010044 dayBegin = new DB0010044();
    dayBegin.BankId = 1;
    dayBegin.BranchId = BranchId;
    dayBegin.DayBeginFlag = 1;
    //added d
    DateTime DayDate = DateTime.ParseExact(Request.Form["LastOpenDate"].ToString(), "dd/MM/yyyy", CultureInfo.InvariantCulture);
    dayBegin.DayDate = DayDate;
    //added d

  //  dayBegin.DayDate = Convert.ToDateTime(Request.Form["LastOpenDate"]);
    dayBegin.DayEndFlag = 0;
    dayBegin.DayEndStage = 1;
    dayBegin.DayReopenFlag = 0;
    OperationStatus status2 = _idDB0010044Repository.AddAndSave(dayBegin);
    if (status2 != null && !status2.Status)
        ModelState.AddModelError("Updation failed", status2.ExceptionMessage);
    else
    {
        CreateInwardSessionsForBranch(BranchId);
        CreateOutwardSessionsForBranch(BranchId);
    }

}

这是错误

会是什么问题？为什么我得到Session null.什么是解决它的方法