DataSet读取XML

string file = File.ReadAllText("c://123.xml", Encoding.Default);
            using (DataSet ds = new DataSet())
            {

                StringReader stream = new StringReader(file);
                ds.ReadXml(stream);
                string a = "";
            }

            using (DataSet ds = new DataSet())
            {
                ds.ReadXml("c://123.xml");
                string a = "";
            }