当您使用像 Entity Framework 或 NHibernate 这样的 ORM 时,它已经被处理了 - 所有读取的实体都通过 IdentityMap 机制进行跟踪,如果实体已经加载,则通过键搜索(EF 中的 DbSet.Find)甚至不会命中数据库.
如果您使用直接数据库访问或 microORM 作为存储库的基础,则应该小心 - 如果没有 IdentityMap,您实际上是在使用值对象:
using System;
using System.Collections.Generic;
using System.Linq;
namespace test
{
internal class Program
{
static void Main()
{
Console.WriteLine("Identity map");
var artrepo1 = new ArticleIMRepository();
var o1 = new Order();
o1.OrderLines.Add(new OrderLine {Article = artrepo1.GetById(1, "a1", 100), Quantity = 50});
o1.OrderLines.Add(new OrderLine {Article = artrepo1.GetById(1, "a1", 100), Quantity = 30});
o1.OrderLines.Add(new OrderLine {Article = artrepo1.GetById(2, "a2", 100), Quantity = 20});
o1.ConfirmOrder();
o1.PrintChangedStock();
/*
Art. 1/a1, Stock: 20
Art. 2/a2, Stock: 80
*/
Console.WriteLine("Value objects");
var artrepo2 = new ArticleVORepository();
var o2 = new Order();
o2.OrderLines.Add(new OrderLine {Article = artrepo2.GetById(1, "a1", 100), Quantity = 50});
o2.OrderLines.Add(new OrderLine {Article = artrepo2.GetById(1, "a1", 100), Quantity = 30});
o2.OrderLines.Add(new OrderLine {Article = artrepo2.GetById(2, "a2", 100), Quantity = 20});
o2.ConfirmOrder();
o2.PrintChangedStock();
/*
Art. 1/a1, Stock: 50
Art. 1/a1, Stock: 70
Art. 2/a2, Stock: 80
*/
Console.ReadLine();
}
#region "Domain Model"
public class Order
{
public List<OrderLine> OrderLines = new List<OrderLine>();
public void ConfirmOrder()
{
foreach (OrderLine line in OrderLines)
{
line.Article.Stock -= line.Quantity;
}
}
public void PrintChangedStock()
{
foreach (var a in OrderLines.Select(x => x.Article).Distinct())
{
Console.WriteLine("Art. {0}/{1}, Stock: {2}", a.Id, a.Name, a.Stock);
}
}
}
public class OrderLine
{
public Article Article;
public int Quantity;
}
public class Article
{
public int Id;
public string Name;
public int Stock;
}
#endregion
#region Repositories
public class ArticleIMRepository
{
private static readonly Dictionary<int, Article> Articles = new Dictionary<int, Article>();
public Article GetById(int id, string name, int stock)
{
if (!Articles.ContainsKey(id))
Articles.Add(id, new Article {Id = id, Name = name, Stock = stock});
return Articles[id];
}
}
public class ArticleVORepository
{
public Article GetById(int id, string name, int stock)
{
return new Article {Id = id, Name = name, Stock = stock};
}
}
#endregion
}
}