In my previous post (Displaying the device picker in a UWP application) I showed how to reuse the UWP device picker flyout within an application.

However, it’s possible that the developer would prefer to offer devices in some alternate UI or, of course, simply locate a known device for an application to use. In the previous post I looked at filtering for Bluetooth devices and those are what I’m after for a little application I’m writing in which I do not require the DevicePicker UI.

Filtering

Before I get into code to enumerate the devices, I’ll first expand on the lines of code from my previous post, that look like this

BluetoothDevice.GetDeviceSelectorFromPairingState(false)

Each of the lines like this one actually returns a string which is an Advanced Query Syntax (AQS) language query. So for example this line will produce the following string (formatted to make it more readable)

System.Devices.DevObjectType:=5 AND 
System.Devices.Aep.ProtocolId:=\"{E0CBF06C-CD8B-4647-BB8A-263B43F0F974}\" AND 
(System.Devices.Aep.IsPaired:=System.StructuredQueryType.Boolean#False OR
System.Devices.Aep.Bluetooth.IssueInquiry:=System.StructuredQueryType.Boolean#True)

For completeness, here’s the results of each of the other GetDeviceSelectorFromPairingState calls

BluetoothLEDevice.GetDeviceSelectorFromPairingState(false)

System.Devices.DevObjectType:=5 AND 
System.Devices.Aep.ProtocolId:=\"{BB7BB05E-5972-42B5-94FC-76EAA7084D49}\" AND 
(System.Devices.Aep.IsPaired:=System.StructuredQueryType.Boolean#False OR 
System.Devices.Aep.Bluetooth.IssueInquiry:=System.StructuredQueryType.Boolean#True)

BluetoothLEDevice.GetDeviceSelectorFromPairingState(true)

System.Devices.DevObjectType:=5 AND 
System.Devices.Aep.ProtocolId:=\"{BB7BB05E-5972-42B5-94FC-76EAA7084D49}\" AND 
(System.Devices.Aep.IsPaired:=System.StructuredQueryType.Boolean#True OR 
System.Devices.Aep.Bluetooth.IssueInquiry:=System.StructuredQueryType.Boolean#False)

BluetoothDevice.GetDeviceSelectorFromPairingState(true)

System.Devices.DevObjectType:=5 AND 
System.Devices.Aep.ProtocolId:=\"{E0CBF06C-CD8B-4647-BB8A-263B43F0F974}\" AND 
(System.Devices.Aep.IsPaired:=System.StructuredQueryType.Boolean#True OR 
System.Devices.Aep.Bluetooth.IssueInquiry:=System.StructuredQueryType.Boolean#False)

Hence we could now create our own AQS query to search for devices.

See Using Advanced Query Syntax Programmatically for more information on AQS.

Enumerating over devices (simple approach)

A simple approach to enumerating over all the available devices is to use the DeviceInformation.FindAllAsync() method, which is async/await compatible, hence we simply use it like this

var devices = await DeviceInformation.FindAllAsync();

// output all devices
foreach (var device in devices)
{
   Debug.WriteLine(device.Name);
}

Obviously this is a little over the top if we’re looking for a specific device or set of devices. In such cases we can create an AQS query and pass this into one of the FindAllAsync overloads. Hence to recreate my previous post’s query looking for all Bluetooth devices we might prefer to use

var devices = await DeviceInformation.FindAllAsync(
    BluetoothDevice.GetDeviceSelectorFromPairingState(
        false));

Whilst this solution to the problem of locating devices may fulfil many of the developer’s requirements, an area it fails on is that once the developer has a list of devices they do not have a way to tell when devices are turned off/disabled/or otherwise no longer available. In such situations it’s better to watch for device changes.

Watching for device changes

The DeviceInformation.CreateWatcher method allows the developer to query for devices and via events, watch for items to be added, updated or removed. This would suit an RX type of implementation.

Here’s some code that demonstrates possible usage of the CreateWatcher

try
{
   deviceWatcher = DeviceInformation.CreateWatcher(
      BluetoothDevice.GetDeviceSelectorFromPairingState(false), 
      null,
      DeviceInformationKind.Device);

      deviceWatcher.Added += (watcher, args) =>
      {
         Debug.WriteLine($"Added {args.Name}");
      };
      deviceWatcher.Updated += (watcher, args) =>
      {
         Debug.WriteLine($"Updated {args.Id}");
      };
      deviceWatcher.Removed += (watcher, args) =>
      {
         Debug.WriteLine($"Removed {args.Id}");
      };
      deviceWatcher.EnumerationCompleted += (watcher, args) => 
      { 
         Debug.WriteLine("No more devices found"); 
      };
      deviceWatcher.Start();
}
catch (ArgumentException ex)
{
   Debug.WriteLine(ex.Message);
}

Again we’re using the AQS created via the GetDeviceSelectoryFromPairingState method and in this simplistic example, we simply subscribe to the Added/Updated/Removed and EnumerationCompleted events to output what devices have been added etc. We also need to be aware of possible ArgumentExceptions, such as incorrectly formatted GUID’s etc. See DeviceInformation Class for a more complete example of this usage.