using System;

using System.Linq;

using System.Xml.Linq;

​

public class Program

{

    public static void Main()

    {

        const string xml =

            @"<PutUserLinkRequest xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"">

  <request xmlns=""http://tempuri.org"">

    <id S=""ID-KMEHR"" SV=""1.0"" SL="""">Delta.PutUserLink.25/08/2017 13:07:43</id>

    <author>

      <hcparty>

      </hcparty>

    </author>

  </request>

  <userlink xmlns=""http://tempuri.org"">

    <user1>

      <cd S=""CD-USERTYPE"" SV=""1.0"" SL="""" DN="""" L=""fr"">patient</cd>

      <patient>

        <id S=""ID-PATIENT"" SV=""1.0"" SL="""">97031038713</id>

      </patient>

    </user1>

    <user2>

    </user2>

    <type S=""CD-USERLINK"" SV=""1.0"" SL="""" DN="""" L=""fr"">patientassent</type>

  </userlink>

</PutUserLinkRequest>";

​

        var doc = XDocument.Parse(xml);

​

        XNamespace ns = "http://tempuri.org";

​

        var id = (string) doc.Descendants(ns + "patient")

            .Elements(ns + "id")

            .Single();

​

        Console.WriteLine(id);

    }

}