string poststr = "<?xml version='1.0' encoding='UTF-8'?>";
              poststr += "<MasMessage xmlns='http://www.xxx.com/interface'>";
              poststr += "  <version>1.0</version>";
              poststr += "  <TxnMsgContent>";
              poststr += "      <txnType>测试测试测试</txnType>";
              poststr += "      <cardNo>888888</cardNo>";
              poststr += "  </TxnMsgContent>";
              poststr += "</MasMessage>";
DataSet ds = new DataSet();
StringReader getsr = new StringReader(poststr );
ds.ReadXml(getsr);