Retrive a XAML text from RichTextBox:

private static string GetRTF(RichTextBox rt)

{

    TextRange range = new TextRange(rt.Document.ContentStart, rt.Document.ContentEnd);

    MemoryStream stream = new MemoryStream();

    range.Save(stream, DataFormats.Xaml);

    string xamlText = Encoding.UTF8.GetString(stream.ToArray());

    return xamlText;

}

Render a XAML text into a RichTextBox:

private static FlowDocument SetRTF(string xamlString)

{

    StringReader stringReader = new StringReader(xamlString);

    XmlReader xmlReader = XmlReader.Create(stringReader);

    Section sec = XamlReader.Load(xmlReader) as Section;

    FlowDocument doc = new FlowDocument();

    while (sec.Blocks.Count > 0)

        doc.Blocks.Add(sec.Blocks.FirstBlock);

    return doc;

}