/// <summary>
/// Turns a string into a properly XML Encoded string.
/// Uses simple string replacement.
/// 
/// Also see XmlUtils.XmlString() which uses XElement
/// to handle additional extended characters.
/// </summary>
/// <param name="text">Plain text to convert to XML Encoded string</param>
/// <param name="isAttribute">
/// If true encodes single and double quotes, CRLF and tabs.
/// When embedding element values quotes don't need to be encoded.
/// When embedding attributes quotes need to be encoded.
/// </param>
/// <returns>XML encoded string</returns>
/// <exception cref="InvalidOperationException">Invalid character in XML string</exception>
public static string XmlString(string text, bool isAttribute = false)
{
 var sb = new StringBuilder(text.Length);

 foreach (var chr in text)
 {
  if (chr == '<')
   sb.Append("&lt;");
  else if (chr == '>')
   sb.Append("&gt;");
  else if (chr == '&')
   sb.Append("&amp;");

  // special handling for quotes
  else if (isAttribute && chr == '\"')
   sb.Append("&quot;");
  else if (isAttribute && chr == '\'')
   sb.Append("&apos;");

  // Legal sub-chr32 characters
  else if (chr == '\n')
   sb.Append(isAttribute ? "&#xA;" : "\n");
  else if (chr == '\r')
   sb.Append(isAttribute ? "&#xD;" : "\r");
  else if (chr == '\t')
   sb.Append(isAttribute ? "&#x9;" : "\t");

  else
  {
   if (chr < 32)
    throw new InvalidOperationException("Invalid character in Xml String. Chr " +
             Convert.ToInt16(chr) + " is illegal.");
   sb.Append(chr);
  }
 }

 return sb.ToString();
}