C# datatable.GetChanges() seems not working with my custom datable (return all rows)

public class ControlPointsDataTable: BaseDataTable
    {
        public ControlPointsDataTable()
        {
            TableName = "ControlPoints";
            TableDescription = "Contains system control points";

        DataColumn dataColumnID = new DataColumn("ID", typeof(System.Int64));
        dataColumnID.Caption = "ID";
        dataColumnID.AllowDBNull  = false;
        dataColumnID.DefaultValue = -1;
        Columns.Add(dataColumnID);

        DataColumn dataColumnName = new DataColumn("Name", typeof(System.Int64));
        dataColumnName.Caption = "CP Name";
        dataColumnName.AllowDBNull  = false;
        dataColumnName.DefaultValue = -1;
        Columns.Add(dataColumnName);

        }

        protected override Type GetRowType()
        {
          return typeof(ControlPointsRow);
        }

        protected override DataRow NewRowFromBuilder(DataRowBuilder builder)
        {
          return new ControlPointsRow(builder);
        }

        public ControlPointsRow this[int idx]
        {
          get { return (ControlPointsRow)Rows[idx]; }
        }

        public void Add(ControlPointsRow row)
        {
          Rows.Add(row);
        }

        public void Remove(ControlPointsRow row)
        {
          Rows.Remove(row);
        }

        public ControlPointsRow GetNewRow()
        {
          ControlPointsRow row = (ControlPointsRow)NewRow();

          return row;
        }

        public class ControlPointsRow : DataRow
        {

        [DataMember]
        public System.Int64 ID 
        {
          get {return (System.Int64)base["ID"];}
          set {base["ID"]=value;}
        }

        [DataMember]
        public System.Int64 Name 
        {
          get {return (System.Int64)base["Name"];}
          set {base["Name"]=value;}
        }


          internal ControlPointsRow(DataRowBuilder builder) : base(builder) {} 
        }
    }

[ServiceContract(Namespace = Constants.NAMESPACE)]
public class BaseDataTable : DataTable
{
    public string TableDescription { get; set; }
}