使用一个非持久字段PersistentAlias

public class Order : XPObject {
   public Order() {}
   public decimal UnitPrice;
   public int Qty;
   public decimal Discount;

   [PersistentAlias("UnitPrice*Qty*(1-Discount)")]
   public decimal ExtendedPrice {
      get { return Convert.ToDecimal(EvaluateAlias("ExtendedPrice")); }
   }
}

        [PersistentAlias("TaskWork[].Sum(HoursSpent)")]
        public float HoursSpent {
            get { return (float)(EvaluateAlias("HoursSpent") ?? 0f); }
        }
        public float TotalHours {
            get { return RemainingHours + HoursSpent; }
        }
        [Association, Aggregated]
        public XPCollection<TaskWork> TaskWork {
            get { return GetCollection<TaskWork>("TaskWork"); }
        }