WPF ListBox with a ListBox – UI Virtualization and Scrolling

It is possible to achieve smooth scrolling VirtualizingStackPanels in WPF 4.0 without sacrificing virtualization if you’re prepared to use reflection to access private functionality of the VirtualizingStackPanel. All you have to do is set the private IsPixelBased property of the VirtualizingStackPanel to true.

Note that in .Net 4.5 there’s no need for this hack as you can set VirtualizingPanel.ScrollUnit=”Pixel”.

To make it really easy, here’s some code:

public static class PixelBasedScrollingBehavior 
{
    public static bool GetIsEnabled(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsEnabledProperty);
    }

    public static void SetIsEnabled(DependencyObject obj, bool value)
    {
        obj.SetValue(IsEnabledProperty, value);
    }

    public static readonly DependencyProperty IsEnabledProperty =
        DependencyProperty.RegisterAttached("IsEnabled", typeof(bool), typeof(PixelBasedScrollingBehavior), new UIPropertyMetadata(false, HandleIsEnabledChanged));

    private static void HandleIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var vsp = d as VirtualizingStackPanel;
        if (vsp == null)
        {
            return;
        }

        var property = typeof(VirtualizingStackPanel).GetProperty("IsPixelBased",
                                                                     BindingFlags.NonPublic | BindingFlags.Instance);

        if (property == null)
        {
            throw new InvalidOperationException("Pixel-based scrolling behaviour hack no longer works!");
        }

        if ((bool)e.NewValue == true)
        {
            property.SetValue(vsp, true, new object[0]);
        }
        else
        {
            property.SetValue(vsp, false, new object[0]);
        }
    }
}

To use this on a ListBox, for example, you would do:

<ListBox>
   <ListBox.ItemsPanel>
      <ItemsPanelTemplate>
         <VirtualizingStackPanel PixelBasedScrollingBehavior.IsEnabled="True">
          </VirtualizingStackPanel>
       </ItemsPanelTemplate>
   </ListBox.ItemsPanel>
</ListBox>