public void deleteAppointment(Appointment st) { var original = Appointments.Find(a => a.AppointmentID == st.AppointmentID); if (original != null && Encoding.ASCII.GetString(original.timestamp) == Encoding.ASCII.GetString(st.timestamp)) { var at = Attendances.First(a => a.AppointmentID == st.AppointmentID); Attendances.Remove(at); } else { throw new Exception("Your appointment version is not actualy"); } }