WPF Dependency Property not being set

Question

I am trying to bind a dependency property via XAML to my custom WPF control.

Here's how I register the dependency property:

public static readonly DependencyProperty AltNamesProperty = 
    DependencyProperty.Register ("AltNames", typeof(string), typeof(DefectImages));

public string AltNames
{
    get { return (string) GetValue(AltNamesProperty); }
    set { SetValue(AltNamesProperty, value); }
}

And Here's how I call it in my XAML:

<DataGrid.Columns>                
    <DataGridTemplateColumn IsReadOnly="True">
        <DataGridTemplateColumn.CellTemplate>
            <DataTemplate>
                <StackPanel Name="StackPanel1" Grid.Column="0" Width="950">
                    <TextBlock FontSize="16" TextDecorations="None" Text="{BindingPath=StandardName}" Foreground="Black"  FontWeight="Bold" Padding="5,10,0,0"></TextBlock>
                    <TextBlock Text="{Binding Path=AltNames}"TextWrapping="WrapWithOverflow" Padding="5,0,0,10"></TextBlock>
                    <!-- this part should be magic!! -->
                    <controls:DefectImages AltNames="{Binding Path=AltNames}"></controls:DefectImages>
                </StackPanel>
            </DataTemplate>
        </DataGridTemplateColumn.CellTemplate>
    </DataGridTemplateColumn>
</DataGrid.Columns>

I know the AltNames property that I'm attempting to bind to is a valid property because I can display it in a textblock just fine. Am I registering the Dependency property incorrectly?

What do I need to do to get the correct value assigned to AltNames in my code behind?

Have you tried `AltNames="{Binding Path=AltNames, Mode=TwoWay}"`? Are there any binding errors in the output window during runtime? — nemesv, Jun 01 '12 at 21:20
@nemesv just tried the two way binding, with no luck. There are no binding errors in the output window. — jacobsimeon, Jun 01 '12 at 21:26
It is likely this is getting confused by 2 properties sharing the same name. Did you try renaming one? The workaround you flagged as an answer seems whacky! — Gusdor, Jun 07 '13 at 10:32
In my case, the property was not set inside my `UserControl`'s constructor, but it was in the `Loaded` event. — Walter Stabosz, Jan 23 '18 at 20:25

score 19 · Accepted Answer · answered Jun 01 '12 at 23:03

Thanks to @Danko for getting me started. I registered a callback to set the value when the property changes.
Here's what I finally ended up with:

private static void OnDefectIdChanged(DependencyObject defectImageControl, DependencyPropertyChangedEventArgs eventArgs)
{
  var control = (DefectImages) defectImageControl;
  control.DefectID = (Guid)eventArgs.NewValue;
}

/// <summary>
/// Registers a dependency property, enables us to bind to it via XAML
/// </summary>
public static readonly DependencyProperty DefectIdProperty = DependencyProperty.Register(
    "DefectID",
    typeof (Guid),
    typeof (DefectImages),
    new FrameworkPropertyMetadata(
      // use an empty Guid as default value
      Guid.Empty,
      // tell the binding system that this property affects how the control gets rendered
      FrameworkPropertyMetadataOptions.AffectsRender, 
      // run this callback when the property changes
      OnDefectIdChanged 
      )
    );

/// <summary>
/// DefectId accessor for for each instance of this control
/// Gets and sets the underlying dependency property value
/// </summary>
public Guid DefectID
{
  get { return (Guid) GetValue(DefectIdProperty); }
  set { SetValue(DefectIdProperty, value); }
}

score 2 · Answer 2 · answered Jun 01 '12 at 21:37

2

Maybe you need to specify the PropertyMetadata argument to DependencyProperty.Register, if the property affects how the control is rendered. For example:

DependencyProperty.Register("AltNames", typeof(string), typeof(DefectImages), 
                              new FrameworkPropertyMetadata( null,
                              FrameworkPropertyMetadataOptions.AffectsRender ) );

answered Jun 01 '12 at 21:37

Danko Durbić

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Tried this, and it's still not going for me. I placed a breakpoint on the setter for the AltNames property and it never gets executed. – jacobsimeon Jun 01 '12 at 22:37
2

Well, the setter is actually never executed, because binding is performed "behind the scenes". You could try [this PropertyMetadata constructor](http://msdn.microsoft.com/en-us/library/ms557327.aspx) to specify a callback method which is executed when the property is changed. Also, check out [How can I debug WPF bindings?](http://bea.stollnitz.com/blog/?p=52). – Danko Durbić Jun 01 '12 at 22:48
that's exactly what I just did. Using the callback to set the value worked for me. Thanks for getting me on the right track. – jacobsimeon Jun 01 '12 at 22:57

WPF Dependency Property not being set

2 Answers2