WPF - Automatic horizontal or vertical spacing in StackPanel and other list like panels

example.xaml

<StackPanel Orientation="Horizontal" foo:Spacing.Horizontal="5">
    <Button>Button 1</Button>
    <Button>Button 2</Button>
</StackPanel>

<StackPanel Orientation="Vertical" foo:Spacing.Vertical="5">
    <Button>Button 1</Button>
    <Button>Button 2</Button>
</StackPanel>

<StackPanel Orientation="Vertical" foo:MarginSetter.Margin="5" foo:MarginSetter.LastItemMargin="5 0">
    <Button>Button 1</Button>
    <Button>Button 2</Button>
</StackPanel>

MarginSetter.cs

using System.Windows;
using System.Windows.Controls;
using JetBrains.Annotations;

namespace Foo
{
    public class MarginSetter
    {
        private static Thickness GetLastItemMargin(Panel obj)
        {
            return (Thickness) obj.GetValue(LastItemMarginProperty);
        }

        [UsedImplicitly]
        public static Thickness GetMargin(DependencyObject obj)
        {
            return (Thickness) obj.GetValue(MarginProperty);
        }

        private static void MarginChangedCallback(object sender, DependencyPropertyChangedEventArgs e)
        {
            // Make sure this is put on a panel
            var panel = sender as Panel;
            if (panel == null) return;

            // Avoid duplicate registrations
            panel.Loaded -= OnPanelLoaded;
            panel.Loaded += OnPanelLoaded;

            if (panel.IsLoaded)
            {
                OnPanelLoaded(panel, null);
            }
        }

        private static void OnPanelLoaded(object sender, RoutedEventArgs e)
        {
            var panel = (Panel) sender;

            // Go over the children and set margin for them:
            for (var i = 0; i < panel.Children.Count; i++)
            {
                UIElement child = panel.Children[i];
                var fe = child as FrameworkElement;
                if (fe == null) continue;

                bool isLastItem = i == panel.Children.Count - 1;
                fe.Margin = isLastItem ? GetLastItemMargin(panel) : GetMargin(panel);
            }
        }

        [UsedImplicitly]
        public static void SetLastItemMargin(DependencyObject obj, Thickness value)
        {
            obj.SetValue(LastItemMarginProperty, value);
        }

        [UsedImplicitly]
        public static void SetMargin(DependencyObject obj, Thickness value)
        {
            obj.SetValue(MarginProperty, value);
        }

        // Using a DependencyProperty as the backing store for Margin.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty MarginProperty =
            DependencyProperty.RegisterAttached("Margin", typeof (Thickness), typeof (MarginSetter),
                new UIPropertyMetadata(new Thickness(), MarginChangedCallback));

        public static readonly DependencyProperty LastItemMarginProperty =
            DependencyProperty.RegisterAttached("LastItemMargin", typeof (Thickness), typeof (MarginSetter),
                new UIPropertyMetadata(new Thickness(), MarginChangedCallback));
    }
}

Spacing.cs

using System.Windows;
using JetBrains.Annotations;

namespace Foo
{
    public class Spacing
    {
        public static double GetHorizontal(DependencyObject obj)
        {
            return (double) obj.GetValue(HorizontalProperty);
        }

        public static double GetVertical(DependencyObject obj)
        {
            return (double) obj.GetValue(VerticalProperty);
        }

        private static void HorizontalChangedCallback(object sender, DependencyPropertyChangedEventArgs e)
        {
            var space = (double) e.NewValue;
            var obj = (DependencyObject) sender;

            MarginSetter.SetMargin(obj, new Thickness(0, 0, space, 0));
            MarginSetter.SetLastItemMargin(obj, new Thickness(0));
        }

        [UsedImplicitly]
        public static void SetHorizontal(DependencyObject obj, double space)
        {
            obj.SetValue(HorizontalProperty, space);
        }

        [UsedImplicitly]
        public static void SetVertical(DependencyObject obj, double value)
        {
            obj.SetValue(VerticalProperty, value);
        }

        private static void VerticalChangedCallback(object sender, DependencyPropertyChangedEventArgs e)
        {
            var space = (double) e.NewValue;
            var obj = (DependencyObject) sender;
            MarginSetter.SetMargin(obj, new Thickness(0, 0, 0, space));
            MarginSetter.SetLastItemMargin(obj, new Thickness(0));
        }

        public static readonly DependencyProperty VerticalProperty =
            DependencyProperty.RegisterAttached("Vertical", typeof (double), typeof (Spacing),
                new UIPropertyMetadata(0d, VerticalChangedCallback));

        public static readonly DependencyProperty HorizontalProperty =
            DependencyProperty.RegisterAttached("Horizontal", typeof (double), typeof (Spacing),
                new UIPropertyMetadata(0d, HorizontalChangedCallback));
    }
}