/// <summary>
/// Append text to a RichTextBox and highlight characters that don't match the reference string.
/// The highlight color is hard-coded as Red. This function goes thru the txtArg char-by-char
/// and sets chars Red if they don't match.
/// </summary>
/// <param name="rtbArg">A RichTextBox</param>
/// <param name="txtArg">Text to append. Characters that don't match the reference string are highlighted.</param>
/// <param name="refArg">Reference string.</param>
static void AppendStringDiff(RichTextBox rtbArg, string txtArg, string refArg)
{
   rtbArg.AppendText(((rtbArg.Text.Length > 0) ? "\n" : string.Empty)); // Append newline.
   
   int beg = rtbArg.Text.Length; // Keep track of where we're starting.

   int maxLen = Math.Max(txtArg.Length, refArg.Length); // handles mis-matched lengths.

   rtbArg.AppendText(txtArg + new string(' ', maxLen - txtArg.Length));

   bool matchPrev=false; // remember last match state.

   for (int i = 0; i <= maxLen; i++)
   {
      // Get chars in the txt string and the reference string.
      // If we're past the string's end, use '\0'
      char refChar = i < refArg.Length ? refArg[i] : '\0';
      char txtChar = i < txtArg.Length ? txtArg[i] : '\0';
      
      bool misMatch = refChar != txtChar; // chars different?

      if (matchPrev && !misMatch) // If chars match again or at end of string, get color.
      {
         rtbArg.SelectionLength = beg+i - rtbArg.SelectionStart;
         rtbArg.SelectionBackColor =  Color.Yellow;
      }

      if (i == maxLen) // at end of string.
      {
         break;
      }

      if (misMatch && !matchPrev) // if chars don't match, mark the beginning of Red.
      {
         rtbArg.SelectionStart = beg+i;
      }
      matchPrev = misMatch; // remember match-state.
   } // for
   rtbArg.SelectionLength = 0;
}

      /// <summary>
      /// Example usage and testing.
      /// Capital letters will appear high-lighted because they don't match the reference string.
      /// </summary>
      void AppendStringDiff_Test()
      {
         AppendStringDiff(richTextBox1, "XaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaX" , "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
         AppendStringDiff(richTextBox1, "bbbbbb THIS STRING IS LONG bbbbbbbbbbbbbbbbbbbbbbbb", "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
         AppendStringDiff(richTextBox1, "ccc THIS STRING IS SHORT  ccccccccccccccccccccccc"  , "cccccccccccccccccccccccccccccccccccccccccccccccccc");
         AppendStringDiff(richTextBox1, "dddddddddddddddddddddddddddddddddddddddddddddddddd" , "dddddddddddddddddddddddddddddddddddddddddddddddddd");
         AppendStringDiff(richTextBox1, "eEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeEeE" , "eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee");
      }

a

This static function will append a string to a RichTextBox while high-lighting characters that don't match a reference string. It's similar to a file-compare GUI that high-lights mis-matched characters.

![alt text](https://docs.google.com/uc?id=0Bw1KoEBfCFrEMmY1ZmVjZjUtMGE4Ny00OWE0LTg1YWItNDkzNzljMWZiYTA2&export=download&authkey=CNOQvbwI&hl=en)

RichTextBox, append a text string and high-light portions of the string that don\'t match a reference string.

C#