WPF

private void SaveToXps(string fileName, RichTextBox rtb)
{
  if (string.IsNullOrEmpty(fileName))
  {
    throw new ArgumentNullException("fileName");
  }
  // If the file doesn't end in .xps, add the extension
  if (Path.GetExtension(fileName).ToLower() != ".xps")
  {
    fileName += ".xps";
  }

  using (FileStream stream = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite))
  {
    TextRange range = new TextRange(rtb.Document.ContentStart, rtb.Document.ContentEnd);
    range.Save(fileName, DataFormats.XamlPackage);
  }
}

private void SaveToXps(string fileName)
      {
          if (string.IsNullOrEmpty(fileName))
          {
              throw new ArgumentNullException("fileName");
          }
          // If the file doesn't end in .xps, add the extension
          if (System.IO.Path.GetExtension(fileName).ToLower() != ".xps")
          {
              fileName += ".xps";
          }

          using (FileStream stream = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite))
          {
              TextRange range = new TextRange(FlowDocRdr.Document.ContentStart, FlowDocRdr.Document.ContentEnd);
              range.Save(stream, DataFormats.XamlPackage);
          }
      }

private void SaveToXps_1(object sender, RoutedEventArgs e)
        {
            SaveToXps("C:\\test1.xaml");
        }