WPF Nested ScrollViewer / ListBox Scrolling

Here’s a quickie. I had an issue where I have a ListBox that will (should) never scroll but I need its item selection capability. It is inside of another ScrollViewer (along with some other controls) and that outer ScrollViewer should be what scrolls. Unfortunately ListBox was eating the MouseWheel events, preventing the outer ScrollViewer from scrolling.

A quick Google search didn’t turn up much. I didn’t want to set IsHitTestVisible=False because like I said, I need the selection ability. I also thought changing the template or subclassing ScrollViewer was a bit drastic.

So instead I came up with the following IgnoreMouseWheelBehavior. Technically it’s not ignoring the MouseWheel, but it is “forwarding” the event back up and out of the ListBox. Check it.

/// <summary>
/// Captures and eats MouseWheel events so that a nested ListBox does not
/// prevent an outer scrollable control from scrolling.
/// </summary>
public sealed class IgnoreMouseWheelBehavior : Behavior<UIElement>
{

  protected override void OnAttached( )
  {
      base.OnAttached( );
      AssociatedObject.PreviewMouseWheel += AssociatedObject_PreviewMouseWheel ;
  }

  protected override void OnDetaching( )
  {
      AssociatedObject.PreviewMouseWheel -= AssociatedObject_PreviewMouseWheel;
      base.OnDetaching( );
  }

  void AssociatedObject_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
  {

      e.Handled = true;

      var e2 = new MouseWheelEventArgs(e.MouseDevice,e.Timestamp,e.Delta);
      e2.RoutedEvent = UIElement.MouseWheelEvent;

      AssociatedObject.RaiseEvent(e2);

  }

}

And here’s how you would use it in XAML.

<ScrollViewer Name="IScroll">
    <ListBox Name="IDont">
        <i:Interaction.Behaviors>
            <local:IgnoreMouseWheelBehavior />
        </i:Interaction.Behaviors>
    </ListBox>
</ScrollViewer>