XAF 如何给記錄增加版本控制？

using System;
using System.ComponentModel;

using DevExpress.Xpo;
using DevExpress.Data.Filtering;

using DevExpress.ExpressApp;
using DevExpress.Persistent.Base;
using DevExpress.Persistent.BaseImpl;
using DevExpress.Persistent.Validation;

namespace DXExample.Module
{
[DefaultClassOptions]
public class PO : BaseObject
{
public PO(Session session)
: base(session)
{
// This constructor is used when an object is loaded from a persistent storage.
// Do not place any code here or place it only when the IsLoading property is false:
// if (!IsLoading){
//    It is now OK to place your initialization code here.
// }
// or as an alternative, move your initialization code into the AfterConstruction method.
}

private string _PONO;
public string PONO
{
get
{
return _PONO;
}
set
{
SetPropertyValue("PONO", ref _PONO, value);
if (!IsLoading)
{

//如果更改订单号，需重置Version
_Version = 0;
}
}
}

private int _Version;
[Custom("AllowEdit","False")]
public int Version
{
get
{
return _Version;
}
set
{
SetPropertyValue("Version", ref _Version, value);
}
}

private string _Note;
public string Note
{
get
{
return _Note;
}
set
{
SetPropertyValue("Note", ref _Note, value);
}
}

private DateTime _CreateOn = DateTime.Now;
[Custom("AllowEdit", "False")]
public DateTime CreateOn
{
get
{
return _CreateOn;
}
set
{
SetPropertyValue("CreateOn", ref _CreateOn, value);
}
}
public override void AfterConstruction()
{
base.AfterConstruction();
Version = 0;
}

private int GetPoNoNewVersion()
{
int ver = 1;
ver = Utility.GetNewVersion<PO>(new BinaryOperator("PONO", PONO), Session, "Version", "CreateOn");
return ver;
}
protected override void OnSaving()
{
base.OnSaving();
_Version = Version != 0 ? _Version : GetPoNoNewVersion();
}
}

public static class Utility
{
/// <summary>
/// Get Version
/// Tonyyang
/// 2011-03-23
/// </summary>
/// <typeparam name="T">BO</typeparam>
/// <param name="criteriaOperator"></param>
/// <param name="session"></param>
/// <param name="propertyName"></param>
/// <param name="sortPropertyName"></param>
/// <returns></returns>
public static int GetNewVersion<T>(CriteriaOperator criteriaOperator, Session session, string propertyName, string sortPropertyName) where T : XPCustomObject
{
int ver = 1;
XPCollection<T> xpc = new XPCollection<T>(session) { Criteria = criteriaOperator };
xpc.Sorting.Clear();
xpc.Sorting.Add(new SortProperty(sortPropertyName, DevExpress.Xpo.DB.SortingDirection.Descending));
if (xpc.IsLoaded != true)
xpc.Load();

ver = xpc.Count == 0 ? ver : (int)xpc[0].GetMemberValue(propertyName) + 1;
return ver;
}
}

}