public virtual List <tbt_MonthlyBillingHistory> GetBillingHistoryPeriodList(string contractCode, string billingOCC, Nullable <System.DateTime> fromDate, Nullable <System.DateTime> toDate)
        {
            BLDataEntities context = new BLDataEntities();

            return(context.GetBillingHistoryPeriodList(contractCode, billingOCC, fromDate, toDate).ToList());
        }