using System;

using System.Linq;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        var stringXML = @"<CstmrPmtStsRpt>

    <GrpHdr>

      <MsgId>21233519</MsgId>

      <CreDtTm>2018-11-29T09:28:00</CreDtTm>

      <InitgPty>

        <Nm>Standard Bank SA</Nm>

        <Id>

          <OrgId>

            <BICOrBEI>SBZAZAJJXXX</BICOrBEI>

          </OrgId>

        </Id>

      </InitgPty>

    </GrpHdr>

    <OrgnlGrpInfAndSts>

      <OrgnlMsgId>Domestic Base-612742-300749</OrgnlMsgId>

      <OrgnlMsgNmId>PAIN.001.001.03</OrgnlMsgNmId>

      <OrgnlCreDtTm>2018-11-29T12:23:06</OrgnlCreDtTm>

      <OrgnlNbOfTxs>1</OrgnlNbOfTxs>

      <OrgnlCtrlSum>17500.00</OrgnlCtrlSum>

      <GrpSts>RJCT</GrpSts>

      <StsRsnInf>

        <Rsn>

          <Cd>NARR</Cd>

        </Rsn>

        <AddtlInf>Duplicate File</AddtlInf>

        <AddtlInf>Error: 6789</AddtlInf>

        <AddtlInf>Not Processed</AddtlInf>

      </StsRsnInf>

    </OrgnlGrpInfAndSts>

  </CstmrPmtStsRpt>";

        XDocument xdoc = XDocument.Parse(stringXML);

        var result = (string)xdoc.Descendants("member")

                             .FirstOrDefault(x => (string)x.Element("name") == "responseCode")

                             ?.Element("value");

        var info = xdoc.Descendants("CstmrPmtStsRpt")

       .Descendants("OrgnlGrpInfAndSts")

       .Descendants("StsRsnInf")                                       

       .SelectMany(r=> r.Descendants("AddtlInf")).ToList();

        Console.WriteLine(info.Count);

    foreach(var x in info)

       Console.WriteLine(x);

    }

}