public override void AddItem(Item item) { ArgumentCheck.IsNotNull(item); Area area = item as Area; WorkCentre wc = item as WorkCentre; if (area == null && wc == null) { string message = string.Format("Unable to add {0} to Area.", item); throw new ArgumentException(message); } if (area != null) { Area1.Add(area); } if (wc != null) { WorkCentre.Add(wc); } }