MouseDragElement behavior with code behind

gistfile1.cs

Canvas designArea = (Canvas)sender;

// Create the rectangle and give it a border and fill.
// Note that a rectangle with no fill requires users to drag from the border only.
Rectangle rectangle = new Rectangle();
rectangle.Height = 120;
rectangle.Width = 90;
rectangle.StrokeThickness = 1;
rectangle.Stroke = new SolidColorBrush(Colors.Black);
rectangle.Fill = new SolidColorBrush(Colors.White);

// Add the MouseDragElementBehavior to the rectangle.
// This rectangle needs to only be moveable within the design area
// so use the ConstrainToParentBounds flag.
Interaction.GetBehaviors(rectangle).Add(
    new MouseDragElementBehavior()
    {
        ConstrainToParentBounds = true
    });

// Get the position of mouse within the parent. This serves as a
// reference point for placing the rectangle.
Point position = e.GetPosition(designArea);

// Set the attached properties for the rectangles position
// relative to the design area.
rectangle.SetValue(
    Canvas.TopProperty,
    position.Y - (rectangle.Height / 2));
rectangle.SetValue(
    Canvas.LeftProperty,
    position.X - (rectangle.Width / 2));

// Now add the new rectangle to the design area canvas.
designArea.Children.Add(rectangle);