XML serialization problem

I have a problem. First, I will describe the classes and then I will describe the problem.

I have a class Task3:

    public class Task3
    {
        private long m_TaskId;
        private string m_CustomerCompanyNumber;
 private GroupType m_GroupType; //GroupType is Enum
        private DateTime m_StartDate;
 
 //CustomFieldData soposed to be abstract and is has 4 sons class - this field is the problem.
        private List<CustomFieldData> m_CustomFieldsData;

 //TaskContainer is a class with 4 string members - I will not add this class for simplicity.
        private TaskContainer m_TaskContainer = null;

        [XmlElement(DataType = "lon ...