gistfile1.cs

[DataContract]
class Order
{
    public DateTime CreatedOn { get; set; }
    public List<OrderRow> Orderrows { get; set; }
}
[CollectionDataContract]
class OrderRow
{
    [DataMember]
    public string Article { get; set; }
    [DataMember]
    public int Qty { get; set; }

}