public static string ToStringAsXml(DataSet ds)
    {
        StringWriter sw = new StringWriter();
        ds.WriteXml(sw, XmlWriteMode.IgnoreSchema);
        string s = sw.ToString(); return s;
    }
    public static string ToStringAsXml(DataTable dt)
    {
        StringWriter sw = new StringWriter();
        dt.WriteXml(sw, XmlWriteMode.IgnoreSchema);
        string s = sw.ToString();
        return s;
    }