Setting XAML attributes gets overwritten at runtime?

Question

When I use the following line, implementing a templated control from a Silverlight class library;

<labelSliderControl:SliderControl x:Name="sldX" Label="X" Value="4" />

I see in the VS Disign view the Slider getting the value of 4. But when I run the app the Slider starts at 0. So when I change the attribute in XAML I can see the change, but I also want the value of 4 during runtime ofcourse. Is the depencyproperty overwriting the value in this case?

Here is the templated control from a Silverlight class library;

<Style TargetType="local:SliderControl">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="local:SliderControl">
                <Grid x:Name="LayoutRoot" Background="White">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto" />
                        <RowDefinition />
                        <RowDefinition Height="Auto" />
                    </Grid.RowDefinitions>
                    <sdk:Label x:Name="lblLabel" Content="" HorizontalContentAlignment="Center" />
                    <Slider x:Name="sldValue" Grid.Row="1" Orientation="Vertical" />
                    <TextBox x:Name="txtValue" Grid.Row="2" MaxWidth="50" MinWidth="50" Text="{Binding ElementName=sldValue, Path=Value, StringFormat=\{0:N2\}}" />
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

[TemplatePart(Name = "sldValue", Type = typeof(Slider)), TemplatePart(Name = "lblLabel", Type = typeof(Label))]
public class SliderControl : Control
{
    private Slider _sliderElement;

    private Slider SliderElement
    {
        get  { return _sliderElement; }
        set  {
            if (_sliderElement != null)
                _sliderElement.ValueChanged -= new RoutedPropertyChangedEventHandler<double>(sldValue_ValueChanged);;

            _sliderElement = value;

            if (_sliderElement != null)
                _sliderElement.ValueChanged += new RoutedPropertyChangedEventHandler<double>(sldValue_ValueChanged);
        }
    }

    public event EventHandler<ValueChangingEventArgs> ValueChanging;

    public static readonly DependencyProperty ValueProperty =
        DependencyProperty.Register("Value", typeof(double), typeof(SliderControl), new PropertyMetadata(0.0, ValuePropertyChanged));

    private static void ValuePropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
    {
        if (obj is SliderControl)
        {
            SliderControl sliderUserControl = (SliderControl)obj;
            if (sliderUserControl.SliderElement != null)
            {
                sliderUserControl.SliderElement.Value = (double)args.NewValue;
            }
        }
    }

    public SliderControl()
    {
        this.DefaultStyleKey = typeof(SliderControl);
    }

    private void sldValue_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
    {
        if (ValueChanging != null)
        {
            ValueChanging(this, new ValueChangingEventArgs(e.OldValue, e.NewValue));
        }
    }

    public override void OnApplyTemplate()
    {
        SliderElement = GetTemplateChild("sldValue") as Slider;
        base.OnApplyTemplate();
    }
}

Answer 1

I don't see a "Value" Property as declared when registering the DependencyProperty.

public double Value
{
   get { return (double)base.GetValue(ValueProperty); }
   set { base.SetValue(ValueProperty, value); }
}

and then bind the property to the control in the constructor:

public SliderControl(){

   InitializeComponent();

   this.sldValue.SetBinding(Slider.ValueProperty, new Binding()
   {
      Source = this,
      Path = new PropertyPath("Value"),
      Mode = BindingMode.TwoWay
   });
}

Answer 2

I got it to work ! I just put the solution of gmcalab in the OnApplyTemplate. Thanks to gmcalab.

    public override void OnApplyTemplate()
    {
        SliderElement = GetTemplateChild("sldValue") as Slider;
        if (SliderElement != null)
        {
            SliderElement.SetBinding(Slider.ValueProperty, new Binding()
            {
                Source = this,
                Path = new PropertyPath("Value"),
                Mode = BindingMode.TwoWay
            });
        }
        base.OnApplyTemplate();
    }