Question

I have a problem with DataContextChanged not being raised on a logical child of my custom Panel control. I’ve narrowed it down to this:

Starting from a wizard-generated WPF application I add:

    private void Window_Loaded( object sender, RoutedEventArgs e )     {         var elt = new FrameworkElement();         this.AddLogicalChild( elt );         DataContext = 42;         Debug.Assert( (int)elt.DataContext == 42 );     } 

As I understand, this works because the DataContext is an inheritable dependency property.

Now, I add event handlers for DataContextChanged both on the Window (this) and its logical child:

    this.DataContextChanged +=          delegate { Debug.WriteLine( 'this:DataContextChanged' ); };     elt.DataContextChanged +=          delegate { Debug.WriteLine( 'elt:DataContextChanged' ); }; 

If I run this, only the first event handler will execute. Why is this? If instead of AddLogicalChild( elt ) I do the following:

this.Content = elt; 

both handlers will execute. But this is not an option in my case – I’m adding FrameworkContentElements to my control which aren’t supposed to be visual children.

What’s going on here? Should I do something more besides AddLogicalChild() to make it work?

(Fortunately, there is a rather simple workaround – just bind the DataContext of the element to the DataContext of the window)

BindingOperations.SetBinding( elt, FrameworkElement.DataContextProperty,              new Binding( 'DataContext' ) { Source = this } ); 

Thank you.

Answer 1

You need to override the LogicalChildren property too:

protected override System.Collections.IEnumerator LogicalChildren {     get { yield return elt; } } 

Of course, you’ll want to return any logical children defined by the base implementation, too.