UWP app can't get data from heart rate monitor, receiving protocol error

Question

I'm trying to make a simple UWP app that displays the value read from a heart rate monitor (Wahoo TICKR FIT). I am able to connect to the monitor, but cannot access the value of the HeartRateMeasurement characteristic. The GattReadResult has a Status of ProtocolError and a Value of null. How do I fix the ProtocolError?

This is all in a plain UWP app using the Visual Studio 2017 template. The plan is to then transfer the working code into a HoloLens project.

Here is the code:

            string btAddr = "F013C3B15603";
            System.UInt64 addrInt = Convert.ToUInt64(btAddr, 16);

            BluetoothLEDevice device = await BluetoothLEDevice.FromBluetoothAddressAsync(addrInt);

            string guidString = "0000180d-0000-1000-8000-00805f9b34fb"; // Armband HeartRate uuid
            Guid guid = new Guid(guidString);
            GattDeviceService gattService = device.GetGattService(guid);

            // Try to access the characteristic we want
            DeviceAccessStatus serviceAccessStatus = await gattService.RequestAccessAsync(); // allowed
            GattOpenStatus openStatus = await gattService.OpenAsync(GattSharingMode.SharedReadAndWrite); // success
            GattCharacteristicsResult results = await gattService.GetCharacteristicsAsync();
            IReadOnlyList<GattCharacteristic> charList = results.Characteristics;
            Debug.WriteLine(charList.Count); // should be 2
            GattCharacteristic heartRateChar = charList[0];
            Debug.WriteLine(heartRateChar.AttributeHandle); // should be 15

            // Try to pull the heart rate data
            GattCommunicationStatus commStatus = await heartRateChar.WriteClientCharacteristicConfigurationDescriptorAsync(GattClientCharacteristicConfigurationDescriptorValue.Notify); // This works
            GattReadResult heartRateResult = await heartRateChar.ReadValueAsync(BluetoothCacheMode.Uncached); // Status is ProtocolError (2)

The issue appears at the very end, at the ReadValueAsync call.

Answer 1

Mike Petrichenko's comment was the solution. The characteristic was not readable, and I needed to create an event handler method for the ValueChanged event to receive the data as it comes in.