public Decimal GetRegisteredBillAmountByPeriod(Int32 companyId, DateTime? beginDate, DateTime? endDate,
                                                Int32? accountId)
 {
     var parcelsManager = new ParcelsManager(this);
     IQueryable<Parcel> billParcels = parcelsManager.GetBillParcelsByPeriodInAccount(companyId, beginDate,
                                                                                     endDate, accountId);
     billParcels = billParcels.Where(bill => bill.OperationDate.HasValue);
     if (billParcels.Any())
         return billParcels.Sum(parcel => parcel.Amount);
     return Decimal.Zero;
 }
        /// <summary>
        /// this method returns the total amount of bills
        /// </summary>
        /// <param name="companyId"></param>
        /// <param name="beginDate"></param>
        /// <param name="endDate"></param>
        /// <param name="accountId"></param>
        /// <returns></returns>
        public Decimal GetBillAmountByPeriodInAccount(Int32 companyId, DateTime? beginDate, DateTime? endDate,
                                                      Int32? accountId)
        {
            var parcelsManager = new ParcelsManager(this);
            Decimal billAmountValue = Decimal.Zero;
            IQueryable<Parcel> billParcels = parcelsManager.GetBillParcelsByPeriodInAccount(companyId, beginDate,
                                                                                            endDate, accountId);
            if (billParcels.Any())
                billAmountValue = billParcels.Sum(parcel => parcel.Amount);

            return billAmountValue;
        }