如何在Silverlight中将字符串（以xaml格式）绑定到RichTextBox的xaml属性？

Xaml

INotifyPropertyChanged

public static class RichTextBoxBinder
{
  #region RichTextBox attached properties

  public static readonly DependencyProperty XamlSourceProperty =
    DependencyProperty.RegisterAttached(
      \"XamlSource\",
      typeof(string),
      typeof(RichTextBox),
      new PropertyMetadata(OnXamlSourcePropertyChanged));

  private static void OnXamlSourcePropertyChanged(
    DependencyObject d,
    DependencyPropertyChangedEventArgs e)
  {
    var rtb = d as RichTextBox;
    if (rtb == null) throw new ArgumentException(
      \"Expected a dependency object of type RichTextBox.\", \"d\");

    string xaml = null;
    if (e.NewValue != null)
    {
      xaml = e.NewValue as string;
      if (xaml == null) throw new ArgumentException(\"Expected a value of type string.\", \"e.NewValue\");
    }

    // Set the xaml and reset selection
    rtb.Xaml = xaml ?? string.Empty;
    rtb.Selection.Select(rtb.ContentStart, rtb.ContentStart);
  }

  #endregion

  public static void SetXamlSource(this RichTextBox rtb, string xaml)
  {
    rtb.SetValue(XamlSourceProperty, xaml);
  }

  public static string GetXamlSource(this RichTextBox rtb)
  {
    return (string) rtb.GetValue(XamlSourceProperty);
  }
}

public string MyRichTextXamlProperty
{
  get
  {
    return
      string.Concat(
        @\"<Section xml:space=\"\"preserve\"\" HasTrailingParagraphBreakOnPaste=\"\"False\"\"\",
        @\" xmlns=\"\"http://schemas.microsoft.com/winfx/2006/xaml/presentation\"\">\",
        @\"<Paragraph FontSize=\"\"11\"\" FontFamily=\"\"Portable User Interface\"\"\",
        @\" Foreground=\"\"#FF000000\"\" FontWeight=\"\"Normal\"\" FontStyle=\"\"Normal\"\"\",
        @\" FontStretch=\"\"Normal\"\" TextAlignment=\"\"Left\"\"><Run Text=\"\"aaa\"\" />\",
        @\"</Paragraph></Section>\"
        );
    // Hints: (Thanks Christoph)
    // 1) Pay special attention that you include the appropriate XML namespaces
    //    e.g. 2nd parameter in string.Concat above.
    // 2) When you have to use resources, they have to be DynamicResource and 
    //    not StaticResource. This is because your resources are only available
    //    at runtime.
  }
}

<UserControl
  xmlns=\"http://schemas.microsoft.com/winfx/2006/xaml/presentation\"
  xmlns:x=\"http://schemas.microsoft.com/winfx/2006/xaml\"
  xmlns:local=\"clr-namespace:MyNamespace\"
  x:Class=\"MyClass\" >
  <Grid>
    <RichTextBox local:RichTextBoxBinder.XamlSource=\"{Binding MyRichTextXamlProperty}\" />
  </Grid>
</UserControl>