Canvas.xy.cs

public static class Canvas
{
    public static readonly DependencyProperty XProperty =
        DependencyProperty.RegisterAttached("X", typeof(double), typeof(Canvas), new PropertyMetadata(default(double), OnXchanged));
    private static void OnXchanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        if (IsChanged(e))
            UpdateX((FrameworkElement)o);
    }
    public static void SetX(FrameworkElement element, double value)
    {
        element.SetValue(XProperty, value);
    }
    public static double GetX(FrameworkElement element)
    {
        return (double)element.GetValue(XProperty);
    }

    public static readonly DependencyProperty XpositionRelativeToProperty =
        DependencyProperty.RegisterAttached("XpositionRelativeTo", typeof(XpositionedRelativeTo), typeof(Canvas), new PropertyMetadata(default(XpositionedRelativeTo), XpositionRelativeToChanged));
    private static void XpositionRelativeToChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        if (IsChanged(e))
            UpdateX((FrameworkElement)o);
    }
    public static void SetXpositionRelativeTo(FrameworkElement element, XpositionedRelativeTo value)
    {
        element.SetValue(XpositionRelativeToProperty, value);
    }
    public static XpositionedRelativeTo GetXpositionRelativeTo(FrameworkElement element)
    {
        return (XpositionedRelativeTo)element.GetValue(XpositionRelativeToProperty);
    }

    public static readonly DependencyProperty YProperty =
DependencyProperty.RegisterAttached("Y", typeof(double), typeof(Canvas), new PropertyMetadata(default(double), OnYchanged));
    private static void OnYchanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        if (IsChanged(e))
            UpdateY((FrameworkElement)o);
    }
    public static void SetY(FrameworkElement element, double value)
    {
        element.SetValue(YProperty, value);
    }
    public static double GetY(FrameworkElement element)
    {
        return (double)element.GetValue(YProperty);
    }

    public static readonly DependencyProperty YpositionRelativeToProperty =
        DependencyProperty.RegisterAttached("YpositionRelativeTo", typeof(YpositionedRelativeTo), typeof(Canvas), new PropertyMetadata(default(YpositionedRelativeTo), YpositionRelativeToChanged));
    private static void YpositionRelativeToChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        if (IsChanged(e))
            UpdateY((FrameworkElement)o);
    }
    public static void SetYpositionRelativeTo(FrameworkElement element, YpositionedRelativeTo value)
    {
        element.SetValue(YpositionRelativeToProperty, value);
    }
    public static YpositionedRelativeTo GetYpositionRelativeTo(FrameworkElement element)
    {
        return (YpositionedRelativeTo)element.GetValue(YpositionRelativeToProperty);
    }
    private static void UpdateX(FrameworkElement fe)
    {
        fe.AddHandler(FrameworkElement.SizeChangedEvent,
            new SizeChangedEventHandler(
                (sender, args) => { if (args.WidthChanged) UpdateX((FrameworkElement)sender); }));
        double x = GetX(fe);
        XpositionedRelativeTo relativeTo = GetXpositionRelativeTo(fe);
        switch (relativeTo)
        {
            case XpositionedRelativeTo.Center:
                x -= fe.ActualWidth / 2;
                break;
            case XpositionedRelativeTo.LeftEdge:
                break;
            case XpositionedRelativeTo.RightEdge:
                x -= fe.ActualWidth;
                break;
            default:
                throw new ArgumentOutOfRangeException();
        }
        System.Windows.Controls.Canvas.SetLeft(fe, x);
    }
    private static void UpdateY(FrameworkElement fe)
    {
        fe.AddHandler(FrameworkElement.SizeChangedEvent,
            new SizeChangedEventHandler(
                (sender, args) => { if (args.HeightChanged) UpdateY((FrameworkElement)sender); }));
        double y = GetY(fe);
        YpositionedRelativeTo relativeTo = GetYpositionRelativeTo(fe);
        switch (relativeTo)
        {
            case YpositionedRelativeTo.Center:
                y -= fe.ActualHeight / 2;
                break;
            case YpositionedRelativeTo.BottomEdge:
                break;
            case YpositionedRelativeTo.TopEdge:
                y -= fe.ActualHeight;
                break;
            default:
                throw new ArgumentOutOfRangeException();
        }
        System.Windows.Controls.Canvas.SetBottom(fe, y);
    }
    private static bool IsChanged(DependencyPropertyChangedEventArgs e)
    {
        return e.NewValue != e.OldValue;
    }
}

public enum XpositionedRelativeTo
{
    Center,
    LeftEdge,
    RightEdge
}
public enum YpositionedRelativeTo
{
    Center,
    TopEdge,
    BottomEdge
}

Sample.xaml

<Canvas Grid.ColumnSpan="2" Grid.Row="1">
    <Ellipse Height="100" Width="100" Fill="DodgerBlue" 
         ap:Canvas.X="100"
         ap:Canvas.XpositionRelativeTo="Center"
         ap:Canvas.Y="50" 
         ap:Canvas.YpositionRelativeTo="Center"/>
</Canvas>