Send write request to BLE device via GATT (UWP)

Question

I have a BLE device (a fitness tracker), which ex shows a message when I send a specific write request to it.

In Android it all works great. The device shows instantly, that it received a message:

BluetoothGattCharacteristic characteristic = ... //connecting to BLE device with specific UUID
byte[] data = new byte[] {(byte)0x01, 0x01};
characteristic.setValue(data);
mBluetoothGatt.writeCharacteristic(characteristic);

But in my UWP app, the BLE device doesn't show any reaction:

var devices = await DeviceInformation.FindAllAsync(GattDeviceService.GetDeviceSelectorFromUuid(new Guid("00001811-0000-1000-8000-00805f9b34fb")), null);
GattDeviceService service = await GattDeviceService.FromIdAsync(devices[0].Id);
var gattCharacteristic = service.GetCharacteristics(new Guid("00002a46-0000-1000-8000-00805f9b34fb")).First();

//Writing request
var writer = new DataWriter();
writer.WriteBytes(new byte[] { 0x01, 0x01 });
await gattCharacteristic.WriteValueAsync(writer.DetachBuffer());

Does anyone has an idea?

Answer 1

It can be that you are not connected or you are using the wrong gattCharacteristic service.

Anyway this is how i successfully write to my device:

private async Task SendValues(byte[] toSend)
{      
  IBuffer writer = toSend.AsBuffer();
  try
  {
     // BT_Code: Writes the value from the buffer to the characteristic.         
     var result = await gattCharacteristic.WriteValueAsync(writer);
     if (result == GattCommunicationStatus.Success)
     {
        //Use for debug or notyfy
        var dialog = new Windows.UI.Popups.MessageDialog("Succes");
        await dialog.ShowAsync();
     }
     else
     {
        var dialog = new Windows.UI.Popups.MessageDialog("Failed");
        await dialog.ShowAsync();
     }
  }
  catch (Exception ex) when ((uint)ex.HResult == 0x80650003 || (uint)ex.HResult == 0x80070005)
  {
     // E_BLUETOOTH_ATT_WRITE_NOT_PERMITTED or E_ACCESSDENIED
     // This usually happens when a device reports that it
     //support writing, but it actually doesn't.
     var dialog = new Windows.UI.Popups.MessageDialog(ex.Message);
     await dialog.ShowAsync();
  }
}