Strips illegal Xml characters

    // strips illegal Xml characters:
    static public String XmlEncode(String S)
    {
        if (S == null)
        {
            return null;
        }
    S = Regex.Replace(S, @"[^\u0009\u000A\u000D\u0020-\uD7FF\uE000-\uFFFD]", "", RegexOptions.Compiled);
        return XmlEncodeAsIs(S);
    }

    // leaves whatever data is there, and just XmlEncodes it:
    static public String XmlEncodeAsIs(String S)
    {
        if (S == null)
        {
            return null;
        }
    StringBuilder sTmp = new StringBuilder();
    using (StringWriter sw = new StringWriter())
    {
    using (XmlTextWriter xwr = new XmlTextWriter(sw))
    {
        xwr.WriteString(S);
        sTmp.Append(sw.ToString());
        xwr.Flush();
        xwr.Close();
    }
    sw.Close();
    }
    return sTmp.ToString();
}