private async void getAvaliableCars(object sender, EventArgs e) { cars.Clear(); carTable.Rows.Clear(); if (reservation.carType != 0 && reservation.dateBegin != null && reservation.dateEnd != null && reservation.dateBegin != "0000-00-00" && reservation.dateEnd != "0000-00-00") { await DatabaseService.getAvaliableCars(reservation).ContinueWith((task) => { cars = task.Result; foreach (Car car in cars) { if (carTable.InvokeRequired) { carTable.Invoke(new Action(() => addCarTableRow(car))); } else { addCarTableRow(car); } } if (carTable.InvokeRequired) { carTable.Invoke(new Action(() => carTable.Show())); } else { carTable.Show(); } }); } else { MessageBox.Show("Uzupełnij wszystkie pola."); } }