private void buttonCloseAll_Click(object sender, EventArgs e) { if (SerialExist) { int deviceHandle = UsbRelayDevice.usb_relay_device_open_with_serial_number(RelaySerial, RelaySerial.Length); //Close all channels int closeResult = UsbRelayDevice.usb_relay_device_close_all_relay_channel(deviceHandle); if (closeResult == 0) { MessageBox.Show("All channels are closed."); } else if (closeResult == 1) { MessageBox.Show("Error!"); } UsbRelayDevice.usb_relay_device_close(deviceHandle); } else { MessageBox.Show("Get serial number first!"); } }