public Guid?ReturnRents() { Receipt overdueReceipt = null; if (MessageBox.Show(ReturningLate() ? ConstStrings.Get("ReturnAndPaymentLateFeeQuestion") : ConstStrings.Get("ReturnQuestion"), ConstStrings.Get("Question"), MessageBoxButton.YesNo, MessageBoxImage.Question) == MessageBoxResult.Yes) { List <Rent> returns = new List <Rent>(); foreach (Rent rent in checkedRents.Keys) { returns.Add(rent); } overdueReceipt = CurrentCustomer.ReturnRents(returns); } checkedRents.Clear(); CheckedRentsChangedSignal = true; CheckedRentsChangedSignal = false; CanReturnItems = false; return(overdueReceipt == null ? null : (Guid?)overdueReceipt.Oid); }