字串值存入XML檔案如何避免"不合法的名稱字元"訊息

參考以下函數

private string StringToXML(string XMLstring)
{
StringWriter sw = new StringWriter();
System.Xml.XmlTextWriter xmlw = new System.Xml.XmlTextWriter(sw);
xmlw.WriteString(XMLstring);
xmlw.Close();
return sw.ToString();
}