In this blog post I present a simple attached behaviour that uses a Thumb control within a Popup to adorn any UI element so that the user can re-size it.

A simple feature that has become quite popular on the web is to attache a small handle to text areas so that the user can resize them, this is useful if a user wants to add a large piece of text to a small comment form for example. Interestingly the Google Chrome browser makes all text areas resizeable by default, which leads to web developers wondering how to turn this feature off for their website. I thought that this was a pretty useful feature, so decided to implement a little attached behaviour that would do the same think for Silverlight:

You can make any element resizable by setting the following attached property:

<TextBox Text="This is a re-sizeable textbox"

The implementation of this behaviour is pretty straightforward. When the Attach property changes, i.e. when it is set on an element a Thumb and a Popup are created:

private static Dictionary<FrameworkElement, Popup> _popups =
    new Dictionary<FrameworkElement, Popup>();

private static void OnAttachedPropertyChanged(DependencyObject d,
  DependencyPropertyChangedEventArgs e)
  FrameworkElement element = d as FrameworkElement;

  // Create a popup.
  var popup = new Popup();

  // and associate it with the element that can be re-sized
  _popups[element] = popup;

  // Add a thumb to the pop-up
  Thumb thumb = new Thumb()
    Style = Application.Current.Resources["MyThumbStyle"] as Style
  popup.Child = thumb;

  // add a relationship from the thumb to the target
  thumb.Tag = element;

  popup.IsOpen = true;

  thumb.DragDelta += new DragDeltaEventHandler(Thumb_DragDelta);
  element.SizeChanged += new SizeChangedEventHandler(Element_SizeChanged);

A static dictionary is used to relate the elements which have this property set o their respective Popup. The Thumb is styled via a Style which is looked up from the application resources. The following style re-templates the Thumb, replacing its visuals with an image:

<Application xmlns=""

    <Style TargetType="Thumb" x:Key="MyThumbStyle">
      <Setter Property="Template">
          <ControlTemplate TargetType="Thumb">
            <Image Source="handle.png"


Whenever the element changes size we need to reposition the Popup so that the thumb always remains on the bottom-right edge of the element:

/// <summary>
/// Handle size changes repositioning the thumb / popup
/// </summary>
private static void Element_SizeChanged(object sender, SizeChangedEventArgs e)
  FrameworkElement element = sender as FrameworkElement;

  // find the popup that hosts the thumb
  var popup = _popups[element];

  // reposition the popup
  FrameworkElement root = Application.Current.RootVisual as FrameworkElement;
  Point elementLocation = GetRelativePosition(element, root);

  var childElement = popup.Child as FrameworkElement;

  // Set where the popup will show up on the screen.
  popup.VerticalOffset = elementLocation.Y + element.ActualHeight - childElement.ActualHeight;
  popup.HorizontalOffset = elementLocation.X + element.ActualWidth - childElement.ActualWidth;


public static Point GetRelativePosition(UIElement element, UIElement otherElement)
  return element.TransformToVisual(otherElement)
                .Transform(new Point(0, 0));

And finally, when the thumb is dragged, we resize the element to which it is associated:

/// <summary>
/// Handles drag events from the thumb, resizing the associated element
/// </summary>
private static void Thumb_DragDelta(object sender, DragDeltaEventArgs e)
  Thumb thumb = sender as Thumb;
  var fe = thumb.Tag as FrameworkElement;

  // set the Width / Height of the element
  if (fe.Width == 0.0 || double.IsNaN(fe.Width))
    fe.Width = fe.ActualWidth;
  if (fe.Height == 0.0 || double.IsNaN(fe.Height))
    fe.Height = fe.ActualHeight;

  // apply the drag deltas
  double newWidth = fe.Width + e.HorizontalChange;
  fe.Width = Math.Max(0, newWidth);

  double newHeight = fe.Height + e.VerticalChange;
  fe.Height = Math.Max(0, newHeight);

And that's it! Nice and simple.

You can download the sourcecode here:

Regards,Colin E.