var result = Common.XmlToObject<List<Istoriya_soglasovaniya_with_files2>>(s1, out err1);

    public class Istoriya_soglasovaniya_with_files2
    {
        [XmlElement(ElementName = "Istoriya_soglasovaniya_ext2")]
        public Istoriya_soglasovaniya_ext2 istoria { get; set; }
    }
    public class Istoriya_soglasovaniya_ext2
    {
        [XmlElement(ElementName = "id")]
        public int id { get; set; }
    }

  private static string ssss0 = "<?xml version=\"1.0\" encoding=\"utf-8\" ?>" +
          "<ArrayOfIstoriya_soglasovaniya_with_files2>" +
            "<Istoriya_soglasovaniya_ext2>" +
              "<id>1</id>" +
            "</Istoriya_soglasovaniya_ext2>" +
          "</ArrayOfIstoriya_soglasovaniya_with_files2>";

static void Main(string[] args)
        {
            try
            {
                var err1 = "";
                var s1 = ssss0;
              

                var result = Common.XmlToObject<List<Istoriya_soglasovaniya_with_files2>>(s1, out err1);

          //result получается пустым, а должен быть в одним объектом внутри..
          //почему ???

out err1);
            }
            catch(Exception exp)
            {
                string s = exp.Message;
            }
        }

        public static T XmlToObject<T>(string xml, out string ErrorMessage)
        {
            ErrorMessage = "";
            StringReader stream = null;
            XmlTextReader reader = null;
            try
            {
                XmlSerializer serializer = new XmlSerializer(typeof(T));
                stream = new StringReader(xml);
                // read xml data
                reader = new XmlTextReader(stream);
                // create reader 
                return (T)serializer.Deserialize(reader);
            }
            catch (Exception exp)
            {
                ErrorMessage = exp.Message; throw;
            }
            finally
            {
                if (stream != null) stream.Close();
                if (reader != null) reader.Close();
            }
        }