As promised, I learned more so I'm sharing.  Last time I figured out how to stuff plain text into the RichTextBox, but that's not very "rich", is it? 

How do I put RTF (rich text format) into a RichTextBox?
It turns out the support for this seems to be through the TextRange class.  TextRange offers a "Load" method which takes a string representing a data format (which you can pick from the DataFormats class) and a stream.  If you currently have a string of RTF formatting, you can stuff the string in a MemoryStream, then pass it to the TextRange.  Here's a handy helper function for it:


        private static void LoadRTF(string rtf, RichTextBox richTextBox) {

            if (string.IsNullOrEmpty(rtf)) {
                throw new ArgumentNullException();
            }

            TextRange textRange = new TextRange(richTextBox.Document.ContentStart, richTextBox.Document.ContentEnd);

            //Create a MemoryStream of the Rtf content
           
            using (MemoryStream rtfMemoryStream = new MemoryStream()) {
                using (StreamWriter rtfStreamWriter = new StreamWriter(rtfMemoryStream)) {
                    rtfStreamWriter.Write(rtf);
                    rtfStreamWriter.Flush();
                    rtfMemoryStream.Seek(0, SeekOrigin.Begin);

                    //Load the MemoryStream into TextRange ranging from start to end of RichTextBox.
                    textRange.Load(rtfMemoryStream, DataFormats.Rtf);
                }
            }
        }

How do I load a file with "rich" content in it?
Here again, you can use the TextRange class to load, but you pass in a FileStream instead of a MemoryStream. 

   private static void LoadFile(string filename, RichTextBox richTextBox) {
            if (string.IsNullOrEmpty(filename)) {
                throw new ArgumentNullException();
            }
            if (!File.Exists(filename)) {
                throw new FileNotFoundException();
            }

            // open the file for reading
            using (FileStream stream = File.OpenRead(filename)) {
                // create a TextRange around the entire document
                TextRange documentTextRange = new TextRange(richTextBox.Document.ContentStart, richTextBox.Document.ContentEnd);
               
                // sniff out what data format you've got
                string dataFormat = DataFormats.Text;
                string ext = System.IO.Path.GetExtension(filename);
                if (String.Compare(ext, ".xaml",true) == 0) {
                    dataFormat = DataFormats.Xaml;
                }
                else if (String.Compare(ext, ".rtf", true) == 0) {
                    dataFormat = DataFormats.Rtf;
                }
                documentTextRange.Load(stream, dataFormat);
            }       
        }

How do I Save a file with "rich" content in it?
TextRange also has a Save!  Here's the code

    private static void SaveFile(string filename, RichTextBox richTextBox) {
            if (string.IsNullOrEmpty(filename)) {
                throw new ArgumentNullException();
            }
          

            // open the file for reading
            using (FileStream stream = File.OpenWrite(filename)) {
                // create a TextRange around the entire document
                TextRange documentTextRange = new TextRange(richTextBox.Document.ContentStart, richTextBox.Document.ContentEnd);

                // sniff out what data format you've got
                string dataFormat = DataFormats.Text;
                string ext = System.IO.Path.GetExtension(filename);
                if (String.Compare(ext, ".xaml", true) == 0) {
                    dataFormat = DataFormats.Xaml;
                }
                else if (String.Compare(ext, ".rtf", true) == 0) {
                    dataFormat = DataFormats.Rtf;
                }
                documentTextRange.Save(stream, dataFormat);
            }
        }

Putting it all together - a very simple editor

Replacing the contents of the Window with this DockPanel, adding the SaveFile, LoadFile helper methods and the event handlers for open and save, we can get a very simple rich text editor.

  <!-- Window1.xaml -->
  <DockPanel>
    <Menu DockPanel.Dock="Top">
      <MenuItem Header="_File">
        <MenuItem Header="_Open File" Click="OnOpenFile"/>
        <MenuItem Header="_Save" Click="OnSaveFile"/>
        <Separator/>
        <MenuItem Header="E_xit" Click="OnExit"/>
      </MenuItem>     
    </Menu>
    <RichTextBox Name="richTextBox1"></RichTextBox>    
  </DockPanel>

        // Window1.xaml.cs
        private void OnExit(object sender, EventArgs e) {
            this.Close();
        }
        private void OnOpenFile(object sender, EventArgs e) {
            Microsoft.Win32.OpenFileDialog ofd = new Microsoft.Win32.OpenFileDialog();
            ofd.Filter = "Text Files (*.txt; *.xaml; *.rtf)|*.txt;*.xaml;*.rtf";
            ofd.Multiselect = false;
            if (ofd.ShowDialog() == true) {
                LoadFile(ofd.SafeFileName, richTextBox1);
            }

        }
        private void OnSaveFile(object sender, EventArgs e) {
            Microsoft.Win32.SaveFileDialog sfd = new Microsoft.Win32.SaveFileDialog();
            sfd.Filter = "Text Files (*.txt; *.xaml; *.rtf)|*.txt;*.xaml;*.rtf";
            if (sfd.ShowDialog() == true) {
                SaveFile(sfd.SafeFileName, richTextBox1);
            }
        }