using System;

using System.Linq;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        var stringXML = @"<methodResponse>

                          <params>

                            <param>

                              <value>

                                <struct>

                                  <member>

                                    <name>originTransactionID</name>

                                    <value>

                                      <string>23915</string>

                                    </value>

                                  </member>

                                  <member>

                                    <name>responseCode</name>

                                    <value>

                                      <i4>0</i4>

                                    </value>

                                  </member>

                                </struct>

                              </value>

                            </param>

                          </params>

                        </methodResponse>";

        XDocument xdoc = XDocument.Parse(stringXML);

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

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

                             ?.Element("value");

​

         Console.WriteLine(result);

    }

}