The app manifest includes a Capabilities section in which you must specify all the capabilities that your app requires. There are three reasons for this:

As with content and markets, you can choose to build one app for the lowest common denominator of device or multiple versions for different device configurations. Or, you can make your app available only for a selected subset of devices. You can achieve the first strategy by simply eliminating any features that do not work on all devices in the market. You can achieve the second by building and publishing multiple XAPs. You can achieve the third by declaring appropriate capabilities in your app manifest.

Another option is to adapt your code dynamically to accommodate the constraints of the device on which it’s running. Some hardware capabilities are optional, notably the gyroscope, compass, and camera (front or back-facing). You can test for the presence of each of these sensors and then take alternative code paths, depending on the results. You can use the same technique for features that the user can arbitrarily enable and disable, such as cellular data connectivity and cellular data roaming. Figure 15 shows the OptionalCapabilities app in the sample code, which makes use of these dynamic-discovery techniques. This app merely displays the test results in the UI.

Figure 15. You can test for optional capabilities and adjust your features accordingly.

The app declares a number of data-bound TextBlock controls for each of the “yes/no” test results.

<TextBlock Text="{Binding Gyroscope, Converter={StaticResource converter}}"/>

Each TextBlock is data-bound to a property on a custom OptionalFeatures class, which exposes bool properties for whether or not the device supports a gyroscope, compass, and so on. The binding also uses a custom BoolToStringConverter for converting the bool values to “yes/no” strings.

public class OptionalFeatures : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (null != handler)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
    private bool gyroscope;
    public bool Gyroscope
    {
        get { return gyroscope; }
        set
        {
            if (gyroscope != value)
            {
                gyroscope = value;
                NotifyPropertyChanged("Gyroscope");
            }
        }
    }
    … other properties omitted for brevity.
}

In this app, when the user taps the app bar button, we gather all the optional capability information via tests such as Gyroscope.IsSupported or DeviceNetworkInformation.IsCellularDataEnabled and feed it into an OptionalFeatures object.

public OptionalFeatures Features { get; set; }
public MainPage()
{
    InitializeComponent();
    Features = new OptionalFeatures();
    DataContext = Features;
}
private void appbarRefresh_Click(object sender, EventArgs e)
{
    Features.Gyroscope = Gyroscope.IsSupported;
    Features.Accelerometer = Accelerometer.IsSupported;
    Features.Compass = Compass.IsSupported;
    Features.PrimaryCamera = Camera.IsCameraTypeSupported(CameraType.Primary);
    Features.FrontCamera = Camera.IsCameraTypeSupported(CameraType.FrontFacing);
    Features.CellData = DeviceNetworkInformation.IsCellularDataEnabled;
    Features.CellRoaming = DeviceNetworkInformation.IsCellularDataRoamingEnabled;
}