public ObservableCollection<House> selectAllRelatedHouse() { if (!isValid(new List<string>() { "prName", "pID" })) return null; ObservableCollection<House> houseList = new ObservableCollection<House>(); string sqlCommand = String.Format("select hID from House where prID={0}", prID); Sql sql = new Sql(); SqlDataReader reader = sql.selectHouseIDByPrID(sqlCommand); if (!reader.HasRows) { sql.closeConnection(); return null; } while (reader.Read()) { int hID = Int32.Parse(reader[0].ToString()); House house = new House(); house.id = hID; house.select(); houseList.Add(house); } sql.closeConnection(); return houseList; }