gistfile1.cs

        public decimal? MonthlyPrice
        {
            get
            {
                try
                {
                    return AddOnRates.Single(x => x.PaymentTerm == PaymentTerm.Monthly).Price;
                } catch(InvalidOperationException ex)
                {
                    throw new InvalidOperationException("Monthly price for " + idAddOn + " was not found.",ex);
                }
            }
        }