Question

What determines the order that multiple DepdencyProperties on the same control get evaluated in?

I am using the Extended WPF Toolkit PropertyGrid and have both SelectedObject and PropertyDefinitions bound:

<extToolkit:PropertyGrid AutoGenerateProperties="False" SelectedObject="{Binding ActiveDataPoint}" PropertyDefinitions="{Binding ActiveDataPoint.Properties}">

The problem is that the OnSelectedObjectChanged fires from the dependency property, and in that changed handler it is referencing PropertyDefinitions, which it is seeing as null. If I comment out the OnSelectedObjectChanged handler then I can see when debugging that OnPropertyDefinitionsChanged is called AFTER the call to OnSelectedObjectChanged.

public static readonly DependencyProperty PropertyDefinitionsProperty = DependencyProperty.Register( "PropertyDefinitions", typeof( PropertyDefinitionCollection ), typeof( PropertyGrid ), new UIPropertyMetadata( null, OnPropertyDefinitionsChanged ) );
public PropertyDefinitionCollection PropertyDefinitions
{
  get
  {
    return ( PropertyDefinitionCollection )GetValue( PropertyDefinitionsProperty );
  }
  set
  {
    SetValue( PropertyDefinitionsProperty, value );
  }
}

private static void OnPropertyDefinitionsChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
{
    Console.Write("I changed!");
}

public static readonly DependencyProperty SelectedObjectProperty = DependencyProperty.Register( "SelectedObject", typeof( object ), typeof( PropertyGrid ), new UIPropertyMetadata( null, OnSelectedObjectChanged ) );
public object SelectedObject
{
  get
  {
    return ( object )GetValue( SelectedObjectProperty );
  }
  set
  {
    SetValue( SelectedObjectProperty, value );
  }
}

private static void OnSelectedObjectChanged( DependencyObject o, DependencyPropertyChangedEventArgs e )
{
  PropertyGrid propertyInspector = o as PropertyGrid;
  if( propertyInspector != null )
    propertyInspector.OnSelectedObjectChanged( ( object )e.OldValue, ( object )e.NewValue );
}

The problem I am facing is discussed on this forum thread, but I am asking a more general WPF question of how I can change the order that these properties are updated.

I have tried having multiple calls to NotifyPropertyChanged in different orders but that doesn’t seem to affect this. Can I cause the order to be different or should I just modify the PropertyGrid so that it will work for either order?

Answer 1

The short answer is that it is all a black box and you should not rely on one being evaluated before or after another. So the best approach would be to modify the PropertyGrid so it works regardless of the order the properties are set.

The long answer is it looks like it depends on how the order that the bindings are specified. So you can do:

<extToolkit:PropertyGrid AutoGenerateProperties="False"
    PropertyDefinitions="{Binding ActiveDataPoint.Properties}"
    SelectedObject="{Binding ActiveDataPoint}"
    >

Instead of:

<extToolkit:PropertyGrid AutoGenerateProperties="False"
    SelectedObject="{Binding ActiveDataPoint}"
    PropertyDefinitions="{Binding ActiveDataPoint.Properties}"
    >

Again, it would be bad practice to rely on this. And this quirk may only work for when the control is initialized. Changes to ActiveDataPoint or the DataContext later, may result in a different order.