Custom Drag and Drop with multiple Canvas-elements

DragAndDrop.xaml

<Window x:Class="WPFTryOut.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Canvas Name="rootCanvas" ZIndex="1">
            <Canvas Name="Canvas2" Width="100" Height="100" ZIndex="0" Background="Aquamarine" Left="30" Top="30" MouseLeftButtonDown="DragStart" MouseMove="Drag" PreviewMouseLeftButtonUp="DragEnd">
                <Rectangle Canvas.Left="10" Canvas.Top="10" Width="20" Height="20" Fill="Blue" MouseLeftButtonDown="DragStart" MouseMove="Drag" PreviewMouseLeftButtonUp="DragEnd"></Rectangle>
                <Rectangle Canvas.Left="40" Canvas.Top="60" Width="20" Height="20" Fill="Red" MouseLeftButtonDown="DragStart" MouseMove="Drag" PreviewMouseLeftButtonUp="DragEnd"></Rectangle>
            </Canvas>
            <Canvas Name="Canvas3" Width="100" Height="100" ZIndex="0" Background="Aquamarine" Left="330" Top="30" MouseLeftButtonDown="DragStart" MouseMove="Drag" PreviewMouseLeftButtonUp="DragEnd">
                
            </Canvas>
        </Canvas>
    </Grid>
</Window>

DragAndDrop.xaml.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;

namespace WPFTryOut
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        bool _captured;
        double _left, _top, _canvasX, _canvasY, _originalLeft, _originalTop;
        UIElement _draggedElement;
        private Canvas _draggedOverCanvas;
        private Canvas _originalParentCanvas;

        private void DragStart(object sender, MouseButtonEventArgs e)
        {
            _draggedElement = (UIElement)sender;
            Mouse.Capture(_draggedElement);
            _captured = true;

            // Store original situation for possible reset
            _originalParentCanvas = (Canvas)LogicalTreeHelper.GetParent(_draggedElement);
            _originalLeft = Canvas.GetLeft(_draggedElement);
            _originalTop = Canvas.GetTop(_draggedElement);

            // store coordinates
            _left = _originalLeft;
            _top = _originalTop;
            _canvasX = e.GetPosition(rootCanvas).X;
            _canvasY = e.GetPosition(rootCanvas).Y;

            // For objects in another canvas, check if it is not the root canvas, than change the parent to the root canvas and update coordinates.
            // While in drag-mode, the element belongs to the root canvas.
            if (_originalParentCanvas != null && _originalParentCanvas != rootCanvas && _originalParentCanvas.Children.Contains(_draggedElement))
            {
                _originalParentCanvas.Children.Remove(_draggedElement);
                rootCanvas.Children.Add(_draggedElement);

                _left += Canvas.GetLeft(_originalParentCanvas);
                _top += Canvas.GetTop(_originalParentCanvas);
            }

            // Do not drag any other objects
            e.Handled = true;
        }

        private void Drag(object sender, MouseEventArgs e)
        {
            if (_captured)
            {
                // Get new mouse position relative to root canvas
                double x = e.GetPosition(rootCanvas).X;
                double y = e.GetPosition(rootCanvas).Y;
                // Change left and top of UIElement by the mouse-movement (new position - old position)
                _left += x - _canvasX;
                _top += y - _canvasY;
                Canvas.SetLeft(_draggedElement, _left);
                Canvas.SetTop(_draggedElement, _top);
                // Store new mouse position
                _canvasX = x;
                _canvasY = y;

                // Highlight (or UNhighlight) the canvas you are dragging over. Also: Store it (as a future target).
                var hitTestResult = VisualTreeHelper.HitTest(rootCanvas, e.GetPosition(rootCanvas));
                if (hitTestResult != null && hitTestResult.VisualHit != null)
                {
                    var hit = hitTestResult.VisualHit as Canvas;
                    if (hit != null && hit != _draggedOverCanvas)
                    {
                        if (_draggedOverCanvas != null)
                            _draggedOverCanvas.Background = Brushes.Aquamarine;
                        _draggedOverCanvas = hit;
                        hit.Background = Brushes.Yellow;
                    }

                }
                else
                {
                    if (_draggedOverCanvas != null)
                        _draggedOverCanvas.Background = Brushes.Aquamarine;
                    _draggedOverCanvas = null;
                }
            }
            e.Handled = true;
        }

        private void DragEnd(object sender, MouseButtonEventArgs e)
        {
            Mouse.Capture(null);
            _captured = false;

            // if there is a target canvas
            if (_draggedOverCanvas != null)
            {
                _draggedOverCanvas.Background = Brushes.Aquamarine;
                // and you are not dragging a canvas itself
                if (_draggedElement.GetType() != typeof(Canvas))
                {
                    // add the element to the new canvas
                    rootCanvas.Children.Remove(_draggedElement);
                    _draggedOverCanvas.Children.Add(_draggedElement);

                    // and update its coordinates
                    var newLeft = _left - Canvas.GetLeft(_draggedOverCanvas);
                    var newTop = _top - Canvas.GetTop(_draggedOverCanvas);
                    Canvas.SetLeft(_draggedElement, newLeft);
                    Canvas.SetTop(_draggedElement, newTop);
                }
                _draggedOverCanvas = null;
            }
            // otherwise reset the element to its original position
            else
            {
                if (_draggedElement.GetType() != typeof(Canvas))
                {
                    // add the element to the original canvas
                    rootCanvas.Children.Remove(_draggedElement);
                    _originalParentCanvas.Children.Add(_draggedElement);

                    // and restore its coordinates
                    Canvas.SetLeft(_draggedElement, _originalLeft);
                    Canvas.SetTop(_draggedElement, _originalTop);
                }
            }
        }
    }
}