How To Convert RichTextBox to String (WPF)

 public string ConvertRichTextBoxContentsToString(System.Windows.Controls.RichTextBox rtb)
        {
            try
            {
                TextRange textRange = new TextRange(rtb.Document.ContentStart,
                    rtb.Document.ContentEnd);
                return textRange.Text.ToString().Trim();
            }
            catch (Exception ex)
            {
                //

            }
            return null;
        }