BluetoothLEDevice.FromIdAsync returning null

问题

This is UWP code for getting BLE devices. Why I get bleDevice == null for some devices? I didn't find any documentation which explains that.

    var devices = await DeviceInformation.FindAllAsync(BluetoothLEDevice.GetDeviceSelector());
    foreach (DeviceInformation di in devices)
    {
        Debug.WriteLine(di.Name);
        BluetoothLEDevice bleDevice = await BluetoothLEDevice.FromIdAsync(di.Id);

        if (bleDevice == null) {
            Debug.WriteLine("--- NULL ----");
            continue;
        }
        Debug.WriteLine(bleDevice.Name);
    }

I noticed that I get bleDevice != null for BLE devices that HAVE exclamation mark with STATUS_DEVICE_POWER_FAILURE in device manager in Windows.

I get bleDevice == null for BLE devices that DO NOT HAVE exclamation mark in device manager.

回答1:

I got the same problem. the previous answer from Chi Lee is in fact good but not enough detailed on how to do it (for non-experts;-) ).

Here the detailed process (assuming you have a c# project under Microsoft visual studio):

1. Double click the properties field under your project: this will open a new tab
2. in the opened tab, select Application in the left then click Package Manifest... button
3. A new tab named "package.appxmanifest" is opened. Select, inside it, the Capabilities tab
4. Check Bluetooth field under capabilities
5. Save and recompile your project.

The BluetoothLEDevice.FromIdAsync(di.Id) will no more return null, provided that you already have paired devices (either in your program or manually in Windows .

回答2:

You have to add Bluetooth capability in your appxmanifest file.

来源：https://stackoverflow.com/questions/39479747/bluetoothledevice-fromidasync-returning-null