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 = "";

            }