问题

你正在使用POCO,你想提高修改跟踪的性能,同时使内存消耗更少.另外,你想通过EF的CodeFirst方式来实现.

解决方案

假设你有一个关于Account(帐户)和相关的Payments(支付)的模型,如Figure 13-7

Figure 13-7. A model with an Account entity and a related Payment

 

首先,本例用EF的CodeFirst方式实现,在Listing 13-16,我们创建实体类:Account和Payment.为达到最优化的修改跟踪性能,我们需要允许EF能用修改跟踪代理类自动地封装我们的实体类,代理能立即通知底层的代理跟踪机构能随时跟踪属性的修改,使用代理,EF一直都能知道你的实体的state(状态).当创建代理时,通知事件会被添加到每个属性的setter方法上,这个过程由Object State Manager(对象状态管理器)完成.当达到以下两个条件时EF会立即创建一个代理类:(1)所有的实体的属性都必须是virtual,(2)任何集合类的导航属性类型必须是Icollection<T>.EF会override满足这两个条件的实体类,并添加必要的修改跟踪装置.

我们的Account和Payment实体类符合这两件条件,如Listing 13-16 所示:

Listing 13-16. Our Entity Classes with Properties Marked as virtual and the Navigation Properties Are of Type

ICollection<T>

public class Account

{

public virtual int AccountId { get; set; }

public virtual string Name { get; set; }

public virtual decimal Balance { get; set; }

public virtual ICollection<Payment> Payments { get; set; }

}

public class Payment

{

public virtual int PaymentId { get; set; }

public virtual string PaidTo { get; set; }

public virtual decimal Paid { get; set; }

public virtual int AccountId { get; set; }

}

接下来,在Listing 13-17 我们创建用CodeFirst方式访问EF功能的途径,DbContext对象

Listing 13-17. DbContext Object

public class Recipe5Context : DbContext

{

public Recipe5Context()

: base("Recipe5ConnectionString")

{

// Disable Entity Framework Model Compatibility

Database.SetInitializer<Recipe6Context>(null);

}

protected override void OnModelCreating(DbModelBuilder modelBuilder)

{

modelBuilder.Entity<Account>().ToTable("Chapter13.Account");

modelBuilder.Entity<Payment>().ToTable("Chapter13.Payment");

}

public DbSet<Account> Accounts { get; set; }

public DbSet<Payment> Payments { get; set; }

}

接下来在项目中添加App.config类,并把Listing 13-18 的代码添加到ConnectionStrings节下

Listing 13-18. Connection String

<connectionStrings>

<add name="Recipe5ConnectionString"

connectionString="Data Source=.;

Initial Catalog=EFRecipes;

Integrated Security=True;

MultipleActiveResultSets=True"

providerName="System.Data.SqlClient" />

</connectionStrings>

接下来Listing 13-19 演示了插入,获取,和更新我们的模型.

Listing 13-19. Inserting, Retrieving, and Updating Our Model

using (var context = new Recipe5Context())

{

var watch = new Stopwatch();

watch.Start();

for (var i = 0; i < 5000; i++)

{

var account = new Account

{

Name = "Test" + i,

Balance = 10M,

Payments = new Collection<Payment> { new Payment { PaidTo = "Test" + (i + 1), Paid = 5M } },

};

context.Accounts.Add(account);

Console.WriteLine("Adding Account {0}", i);

}

context.SaveChanges();

watch.Stop();

Console.WriteLine("Time to insert: {0} seconds", watch.Elapsed.TotalSeconds.ToString());

}

using (var context = new Recipe5Context())

{

var watch = new Stopwatch();

watch.Start();

var accounts = context.Accounts.Include("Payments").ToList();

watch.Stop();

Console.WriteLine("Time to read: {0} seconds", watch.Elapsed.TotalSeconds.ToString());

watch.Restart();

foreach (var account in accounts)

{

account.Balance += 10M;

account.Payments.First().Paid += 1M;

}

context.SaveChanges();

watch.Stop();

Console.WriteLine("Time to update: {0} seconds", watch.Elapsed.TotalSeconds.ToString());

}

它是如何工作的

EF的最后一个版本,包含6.0,我们用POCO类来代表实体类,POCO是Plain Old CRL Object缩写,通常只包含与数据库表列对应的态和属性.POCO类没有依赖.NET CLR基类,尤其没有依赖EF的东西.

第二个方式,如Listing 13-19描述的,用一个实现了IentityWithChangeTracking接口的代理对象来包装底层的实体POCO对象.该代理负责通知Object State Manager(对象状态管理器)实体对象的属性和相关联对象属性的修改.EF会自动为你的POCO对象创建代理,当你把在你的POCO类里把每个属性标为virtual,把每个导航属性返回Icollection<T>类型.代理避免了可能复杂的快照方式下的对象与对象之间的比较,虽然代理也需要为跟踪每个变化的发生付出些负载.

尽管修改跟踪代理立即通知修改跟踪组件对象的修改并避免对象的比较,在实践中,性能上所带来的好处也只在模型非常复杂并只有少量的修改时,才能体现出来.Figure 13-7所示的模型非常简单,如果你改变Listing 13-19的代码,用快照的方式,你可能会注意到用代理类只快一两秒.