Control the ZIndex of elements in a Canvas that is the ItemsPanel for an ItemsControl

<ItemsControl.ItemContainerStyle>
            <Style>
                <Setter Property="Canvas.Left" Value="{Binding Position.X}" />
                <Setter Property="Canvas.Top" Value="{Binding Position.Y}" />
                <Setter Property="Canvas.ZIndex" Value="{Binding ZIndex}" />
            </Style>
        </ItemsControl.ItemContainerStyle>

<Window
    x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Name="MyMainWindow">
    <StackPanel>
        <Canvas Width="300" Height="300" Background="Blue">
            <Ellipse Name="green" Fill="Green" Width="200" Height="100" />
            <Ellipse Name="red" Fill="Red" Width="200" Height="100" Canvas.Left="100" Canvas.Top="50" />
        </Canvas>
        <Button Click="Button_Click">Swap</Button>
    </StackPanel>
</Window>

using System.Windows;
using System.Windows.Controls;
namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            RedIndex = 1;
            GreenIndex = 2;
        }

        private int redIndex;
        private int greenIndex;
        public int RedIndex
        {
            get
            {
                return redIndex;
            }
            set
            {
                if (redIndex != value)
                {
                    redIndex = value;
                    Canvas.SetZIndex(red, redIndex);
                }
            }
        }
        public int GreenIndex
        {
            get
            {
                return greenIndex;
            }
            set
            {
                if (greenIndex != value)
                {
                    greenIndex = value;
                    Canvas.SetZIndex(green, greenIndex);
                }
            }
        }
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            int index = RedIndex;
            RedIndex = GreenIndex;
            GreenIndex = index;
        }

    }
}