Bindings on child dependency object of usercontrol not working

Question

I'm trying to get a binding to work on a child object of a user control. The Xaml looks like this:

<MyGrid>
    <MyColumn ExtendedColumnData="{Binding ColumnToolTipDescriptions}"/>
</MyGrid>

Here is how the classes are defined:

[ContentProperty("Columns")]
public class MyGrid : UserControl
{
    private MyColumnCollection _columns;

    [DesignerSerializationVisibility(DesignerSerializationVisibility.Content), Category("Data")]
    public MyColumnCollection Columns
    {
        get
        {
            if (_columns == null)
                _columns = new MyColumnCollection();

            return _columns;
        }
    }
}

public class MyColumnCollection : ObservableCollection<MyGridColumn>
{
}

public class MyGridColumn : DependencyObject
{
    public object ExtendedColumnData
    {
        get { return (object)GetValue(ExtendedColumnDataProperty); }
        set { SetValue(ExtendedColumnDataProperty, value); }
    }

    public static readonly DependencyProperty ExtendedColumnDataProperty =
        DependencyProperty.Register("ExtendedColumnData", typeof(object), typeof(MyGridColumn), new UIPropertyMetadata(null));
}

From what I can tell, the binding is not even attempting to get the data as I've tried putting a converter against the binding, and the breakpoint on the Convert method never gets hit.

I'm using the MVVM pattern so the window's DataContext property is set to a view model.

I've read some other questions on here and tried various permutations of the binding such as:

<MyColumn ExtendedColumnData="{Binding DataContext.ColumnToolTipDescriptions, ElementName=MyViewName}" />
<MyColumn ExtendedColumnData="{Binding DataContext.ColumnToolTipDescriptions, RelativeSource={RelativeSource AncestorType={x:Type local:MyView}}" />

But still no luck, the binding doesn't fire! The annoying thing is, this seems to work fine (if I add the property to the grid):

<MyGrid ExtendedColumnData="{Binding ColumnToolTipDescriptions}">
    <MyColumn />
</MyGrid>

I'm not that experienced with WPF so I'm sure I'm missing something?

Answer 1

The problem is that MyColumnCollection is not inheriting data context (usual properties of a control are not part of inheritance context). If you don't have a data context bindings will not work.

To fix that, try inheriting MyColumnCollection not from ObservableCollection, but from FreezableCollection (freezable properties are part of inheritance context).