De\Serializing list of objects XML

I have been tried to do serialize to list of object. all I get in the XML file is :

<?xml version="1.0" encoding="utf-8"?>
<ArrayOfObserver xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
  <Observer />
  <Observer />
</ArrayOfObserver>

it shows that the observer is empty but the list that i send to him has 2 object. why it happen ? what to do ? what wrongs ?

My code :

in the main :

Serialize(lstObservers);

 public void Serialize(List<Observer> list)
        {
           XmlSerializer serializer = new XmlSerializer(typeof(List<Observer>));
            using ( TextWriter writer = new StreamWriter( @"C:\Users\user\Desktop\MapSample\bin\Debug\ListObserver.xml"))
            {
                serializer.Serialize(writer, list);
            }  
        }

the class observer :

    [Serializable()]
    public class Observer : MapObject
    {
        private int ID_Observer { get; set; }
        private double azimuth;
        private double Long;
        private double Lat;
        private double Lenght;
        private bool haveConnection;
        private bool DrawAzimuth;
        private XmlSerializer ser;

        /// <summary>
        /// C'tor
        /// </summary>
        public Observer(int ID_Observer = 0, double azimuth = 0, double Lat = 0, double Long = 0, double Lenght = 0, bool haveConnection = true, bool DrawAzimuth = true)
        {
            this.ID_Observer = ID_Observer;
            this.azimuth = azimuth;
            this.Long = Long;
            this.Lat = Lat;
            this.haveConnection = haveConnection;
            this.DrawAzimuth = DrawAzimuth;
            this.Lenght = Lenght;
        }

        public Observer()
        {
            ser = new XmlSerializer(this.GetType());
        }
}

EDIT : my propose is to save list of objects.. if anyone have another idea i will be more than happy to learn. Thanks