public void Can_calculate_giftCard_remainingAmount() { var gc = new GiftCard() { Amount = 100 }; gc.GiftCardUsageHistory.Add ( new GiftCardUsageHistory() { UsedValue = 30 } ); gc.GiftCardUsageHistory.Add ( new GiftCardUsageHistory() { UsedValue = 20 } ); gc.GiftCardUsageHistory.Add ( new GiftCardUsageHistory() { UsedValue = 5 } ); gc.GetGiftCardRemainingAmount().ShouldEqual(45); }