Question

I am designing a control that can have two main states. The state will be set via a dependence property. I would like to set the property via XAML. I know that the setter will only run the setValue code if it is set via XAML as explained here: WPF: XAML property declarations not being set via Setters?

So I’ve tried to set up a DependencyPropertyChangedEvent event, but when I run my code I get this error:

XamlParseException Occurred
'The invocation of the constructor on type 'RouteOneClient.VehicleInfoControl'
that matches the specified binding constraints threw an exception.' Line number 
'134' and line position '34'.

When I look through that, it tells me that I cannot create a new instance of my control in my event. The C# logic looks like this:

    public enum VehicleType
    {
        Sale = 1,
        Trade = 2,
    }

    public static readonly DependencyProperty VehicleTypeProperty =  DependencyProperty.Register(
        "VehicleTYPE", 
        typeof(VehicleType), 
        typeof(VehicleInfoControl), 
        new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender, new PropertyChangedCallback(OnVehTypeChanged)));  // optionally metadata for defaults etc.

    public VehicleInfoControl()
    {
        InitializeComponent();
    }

    public VehicleType VehicleTYPE
    {
        get { return (VehicleType)GetValue(VehicleTypeProperty); }
        set
        {
            SetValue(VehicleTypeProperty, value);
        }          
    }

    private static void OnVehTypeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        VehicleInfoControl instance = d as VehicleInfoControl;

        if (instance.VehicleTYPE == VehicleType.Sale)
        {
            instance.TitleLabel.Content = "Sale Vehicle";
            instance.IntendedUse.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleNewUsed.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleVin.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleInceptionMiles.Visibility = System.Windows.Visibility.Visible;
            instance.IntendedUseLabel.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleNewUsedLabel.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleVinLabel.Visibility = System.Windows.Visibility.Visible;
            instance.VehicleInceptionMilesLabel.Visibility = System.Windows.Visibility.Visible;
        }
        else
        {
            instance.TitleLabel.Content = "Trade Vehicle";
            instance.IntendedUse.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleNewUsed.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleVin.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleInceptionMiles.Visibility = System.Windows.Visibility.Hidden;
            instance.IntendedUseLabel.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleNewUsedLabel.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleVinLabel.Visibility = System.Windows.Visibility.Hidden;
            instance.VehicleInceptionMilesLabel.Visibility = System.Windows.Visibility.Hidden;
        }
    }

And the XAML:

<Expander Header="Vehicle Information" Width="Auto" Height="Auto" Grid.Row="2">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width=".3*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <custom:VehicleInfoControl x:Name="SaleVeh" Width="Auto" Height="Auto" Grid.Column="0" VehicleTYPE="Sale"/>
        <custom:VehicleInfoControl x:Name="TradeVeh" Width="Auto" Height="Auto" Grid.Column="2" VehicleTYPE="Trade"/>

    </Grid>
</Expander>

The error given above points to the line.

I feel like I’m following the form correctly, but it isn’t running. Any ideas?

Answer 1

The type of your dependency property is an enum, which is a ValueType, but you specify null as default property value in your property metadata. That won’t work, since null is not a valid value for a ValueType.