Błąd przy serializacji

There was an error reflecting type 'System.Collections.Generic.List

    [Serializable]
    [XmlRoot("")]
    public class AA
    {
        public AA()
        {
        }

        [XmlAttribute("Name")]
        public string Name { get; set; }

        [XmlArray("arr")]
        [XmlArrayItem("arritem", typeof(xx))]
        public List<xx> xxlist = new List<xx>();
    }

    [Serializable]
    [XmlRoot("xx")]
    public class xx
    {
        public xx()
        {
        }

        [XmlElement("myenum")]
        public MyEnum enum= MyEnum.None;

        [XmlElement("1")]
        public long 1{ get; set; }

        [XmlElement("2")]
        public DateTime 2= DateTime.Now;

There was an error reflecting type 'AA' //z pierwszego posta

at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, Boolean repeats, Boolean openModel, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportElement(TypeModel model, XmlRootAttribute root, String defaultNamespace, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(Type type, XmlRootAttribute root, String defaultNamespace)
at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
[..]

 
  private  List<xx>_xxlist = new List<xx>();
  public List<xx> xxlist 
 {
 get {..}
 set {..}
 }