GetDescriptor() null value / 128-bit UUID descriptor? - ios

Question

I'm writing my code using Xamarin. I'm developing my IOS app which allows me to read the BLE devices, Services, characteristic value and activation of the notification.
My BLE beacon have one custom Services that contain two custom characteristics and both have the notification implemented using CCCD.
My ble devices work correctly I test it with BLE scanner app and it working well without any problem.
I can read value and I can active the notification for both characteristic. See picture here.
The app that I wrote using xamarin work correctly (reading services, characteristic value....) the only problem that didn't work is the activation of the notification. Here's a portion of the code :
public UUID Charac_UUID0 = UUID.FromString("0000beef-1212-efde-1523-785fef13d123");
public UUID Charac_UUID = UUID.FromString("0000b1e0-1212-efde-1523-785fef13d123") ;
public UUID Descr_UUID = UUID.FromString("00002902-1212-efde-1523-785fef13d123");
protected BluetoothGattCharacteristic _charac;
....
....
this._charac = App.Current.State.SelectedService.GetCharacteristic(Charac_UUID0);
BluetoothLEManager.Current.ConnectedDevices[App.Current.State.SelectedDevice].SetCharacteristicNotification(_charac, true);
BluetoothGattDescriptor descriptor = _charac.GetDescriptor(Descr_UUID0);
descriptor.SetValue(BluetoothGattDescriptor.EnableNotificationValue.ToArray());
BluetoothLEManager.Current.ConnectedDevices[App.Current.State.SelectedDevice].WriteDescriptor(descriptor);
The code always give me an error at descriptor.SetValue and it indicate me that the descriptor is NULL meaning that _charac.GetDescriptor didn't return any value.
I suspect the Descriptor UUID value (Descr_UUID) is not correct. I don't know excatly how can I determine the Descr_UUID but I saw many example in the internet of people replacing the custom UUID of the caracteristic by 2902 which give me in my case a 128 descriptor UUID equal to 00002902-1212-efde-1523-785fef13d123.
But there is a problem here. The descriptor UUID for both characteristic will be the same because the base UUID is the same for both characteristic?
Any solution?

Answer 1

3 years later im sure you figured it out but, 2902 is the 16 bit value for a Client Characteristic Configuration Descriptor, and I believe it should also be inflated using the "BASE UUID" not just whatever your characteristic descriptor uses. See this: answer
This will convert your 16 (or 32) bit uuids to full 128 bit uuids using the BASE UUID:
public static UUID ConvertUuid(uint uuid)
{
const long msbMask = 0x0000000000001000;
const ulong lsb = 0x800000805f9b34fb;
var msb = msbMask | ((ulong)(uuid & uint.MaxValue) << 32);
return new UUID((long)msb, unchecked((long)lsb));
}