How to: Fill a property type Collection?

public class Order
{
     public string orderNumber {get; set; }
     public DateTime orderDate {get; set; }
     private DateTime RequiredDate {get; set; }
     private DateTime ShippedDate {get; set; }
     public int CustomerNumber {get; set; }
     public bool status {get; set; }
     public string comments {get; set; }
     private IEnumerable<orderdetail> OrderDetail;

     public virtual IEnumerable<orderdetail> OrderDetails
     {
         get {return OrderDetail; }
         set {OrderDetail = value; }
     }

     public Order ()
     {
         OrderDetail = new List<orderdetail> ();
     }
}