using System;

using System.Linq;

using System.Xml.Linq;

​

public class Program

{

    public static void Main()

    {

        const string xml = @"

<createTransactionResponse xmlns=""AnetApi/xml/v1/schema/AnetApiSchema.xsd"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"">

   <messages>

      <resultCode>Ok</resultCode>

      <message>

         <code>I00001</code>

         <text>Successful.</text>

      </message>

   </messages>

   <transactionResponse>

      <responseCode>1</responseCode>

      <authCode>25C10X</authCode>

      <messages>

         <message>

            <code>1</code>

            <description>This transaction has been approved.</description>

         </message>

      </messages>

  </transactionResponse>

</createTransactionResponse>";

​

        XNamespace ns = "AnetApi/xml/v1/schema/AnetApiSchema.xsd";

​

        var root = XElement.Parse(xml);

​

        var text = (string) root.Elements(ns + "messages")

            .Descendants(ns + "text")

            .Single();

​

        Console.WriteLine(text);

    }

}