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.

Answer 1

The getter and setter for a DependencyProperty are not called by anyone except you!

They are simply helpers generated for your benefit (to save you lots of casting and calls to GetValue/SetValue).

All work in DPs (animation, binding etc) happens directly via calls to SetValue and GetValue.

If you need to catch a change in value, provide the change callback in the Register call.

Note: Attached properties (i.e. using RegisterAttached instead of Register) actually do use the setter, while parsing the XAML, but that is another story as normal DPs do not.

Update:

You have not set a default value in your Register, so it defaults to 0 (which is your enum value “Normal”). If you set that as the first value in your binding there is not change so it won’t trigger.

One Solution: Add a “none” enum as your first 0 value, so that “Normal” (value 1) will cause a change. Alternatively set a default value in the Register that is not “Normal”.