DependencyProperty binding not happening on initial load

Question

I'm trying to do something simple -- make a DependencyProperty and then bind to it. However, the getter doesn't appear to fire when I start up the app. (I'm guessing the solution will make me smack my head and go "Doh!", but still. :) )

Any ideas?

Code-behind code:

public static readonly DependencyProperty PriorityProperty = 
        DependencyProperty.Register("Priority", 
        typeof (Priority), typeof (PriorityControl), null);

public Priority Priority
{
    get { return (Priority)GetValue(PriorityProperty); }
    set { SetValue(PriorityProperty, value); }
}

Control XAML:

<ListBox Background="Transparent" 
         BorderThickness="0" 
         ItemsSource="{Binding Path=Priorities}" 
         Name="PriorityList" 
         SelectedItem="{Binding Path=Priority, Mode=TwoWay}">
  <ListBox.ItemTemplate>
    <DataTemplate>
        <Grid Height="16" Width="16">
          <Border BorderBrush="Black"
                  BorderThickness="2"
                  CornerRadius="3"
                  Visibility="{Binding RelativeSource=
                       {RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected, Converter={StaticResource boolToVisibilityConverter}}" />
          <Border CornerRadius="3" Height="12" Width="12">
            <Border.Background>
              <SolidColorBrush Color="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=Content, Converter={StaticResource priorityToColorConverter}}" />
            </Border.Background>
          </Border>
        </Grid>
    </DataTemplate>
   </ListBox.ItemTemplate>
   <ListBox.ItemsPanel>
     <ItemsPanelTemplate>
       <StackPanel Orientation="Horizontal"/>
     </ItemsPanelTemplate>
   </ListBox.ItemsPanel>
 </ListBox>

Binding statement:

<UI:PriorityControl Grid.Column="8" 
                    Priority="{Binding Path=Priority}" 
                    VerticalAlignment="Center"/>

Some other notes:

Binding is in a UserControl
UserControl contains the PriorityControl
PriorityControl contains the DependencyProperty
I've checked that the data the UserControl is getting the appropriate data -- every other binding works.
If I change the selection on the PriorityControl via the mouse, everything fires as appropriate. It's just that initial setting of the value that isn't working.
Priority is an enum.

EDIT: I've tried two additional things. First, I made the binding to the value two-way, but that didn't work. Then, I added a property changed callback to the dependency property and made it call OnPropertyChanged, like so:

public static readonly DependencyProperty PriorityProperty =
        DependencyProperty.Register("Priority", typeof (Priority), typeof (PriorityControl), new PropertyMetadata(HandleValueChanged));

private static void HandleValueChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
{
   var npc = dependencyObject as PriorityControl;
   npc.OnPropertyChanged("Priority");
}

That didn't work either. I even tried overriding ApplyTemplate on the control to force a property change notification, but the value is never getting set on initial load. I can change values and see everything fire just fine, but it's that first time that nothing is coming through.

Jeff Winn Jeff Winn · Accepted Answer · 2012-06-07T19:14:13

I've noticed that when doing dependency properties if the value is already been set during the definition of the property it won't act correctly. You have said that there were no problems when setting the value within the application, so I'm going to assume there is no plumbing problems.

Consider the example below:

XAML

<UI:PriorityControl Priority="{Binding Path=Priority, Mode=TwoWay}"/>

Code

public enum Priority
{
    Normal,
    Low,
    High
}

public class PriorityControl : UserControl
{    
    public static readonly DependencyProperty PriorityProperty =
      DependencyProperty.Register("Priority", typeof(Priority), typeof(PriorityControl), new PropertyMetadata(DependencyProperty.UnsetValue, OnPriorityPropertyChanged);

    public Priority Priority
    {
        get { return (Priority)this.GetValue(PriorityProperty); }
        set { this.SetValue(PriorityProperty, value); }
    }

    private static void OnPriorityPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        PriorityControl control = sender as PriorityControl;
        if (control != null)
        {
            // Do what you wanted to here.
        }
    }
}

If you're using an enum, and not specifying a default value (which you appear not to be by not providing a PropertyMetadata object to the dependency property on creation) the default value would be that of the enum's default value. In my above example that would be "Normal". Now if you tried to set the value to Normal on creating the control, the callback would not be called because the value already equals that value.

In order to allow this to work correctly, you must pass the DependencyProperty.UnsetValue to the PropertyMetadata object as I have done when I defined the property. That will allow the OnPriorityPropertyChanged method to be called when the value is being set initially.

Hope that helps fix it!

DependencyProperty binding not happening on initial load

3 Answers

Update: