实体框架懒惰加载不能从其他线程工作 [英] Entity Framework lazy loading doesn't work from other thread

问题描述

我刚刚发现，实体框架中的延迟加载只能从创建 ObjectContext 的线程工作。为了说明问题，我做了一个简单的测试，一个简单的模型只包含两个实体： Person 和 Address 。以下是代码：

  private static void TestSingleThread（）
 {
 using（var context = new TestDBContext （））
 {
 foreach（var p in context.Person）
 {
 Console.WriteLine（{0}住在{1}，p.Name， p.Address.City）; 
} 
} 
} 
 
 private static void TestMultiThread（）
 {
 using（var context = new TestDBContext（））
 {
 foreach（var p in context.Person）
 {
 Person p2 = p; //以避免捕获循环变量
 ThreadPool.QueueUserWorkItem（
 arg => 
 {
 Console.WriteLine（{0}住在{1}，p2 .Name，p2.Address.City）; 
}）; 
} 
} 
} 
  

TestSingleThread 方法工作正常，地址属性被延迟加载。但是在 TestMultiThread 中，我得到一个 NullReferenceException 在 p2.Address.City ，因为 p2.Address 为null。

这是一个bug？这是应该工作的方式吗？如果是这样，有没有提到的文件？我在MSDN或Google上找不到任何内容。

更重要的是，是否有解决方法？ （除了从工作线程显式调用 LoadProperty ...）

任何帮助将非常感谢

PS：我使用的是VS2010，所以是EF 4.0。我不知道在以前版本的EF中是否一样。

解决方案

这是按设计吗？是;任何调用Load（隐式或者显式）的调用最终都将通过 ObjectContext 和 ObjectContext被记录为不是线程安全的。

可能的解决方法要将实体与工作线程中的对象上下文分离，并将其附加到当前线程中的对象上下文中。

I just found out that lazy loading in Entity Framework only works from the thread that created the ObjectContext. To illustrate the problem, I did a simple test, with a simple model containing just 2 entities : Person and Address. Here's the code :

    private static void TestSingleThread()
    {
        using (var context = new TestDBContext())
        {
            foreach (var p in context.Person)
            {
                Console.WriteLine("{0} lives in {1}.", p.Name, p.Address.City);
            }
        }
    }

    private static void TestMultiThread()
    {
        using (var context = new TestDBContext())
        {
            foreach (var p in context.Person)
            {
                Person p2 = p; // to avoid capturing the loop variable
                ThreadPool.QueueUserWorkItem(
                    arg =>
                    {
                        Console.WriteLine("{0} lives in {1}.", p2.Name, p2.Address.City);
                    });
            }
        }
    }

The TestSingleThread method works fine, the Address property is lazily loaded. But in TestMultiThread, I get a NullReferenceException on p2.Address.City, because p2.Address is null.

It that a bug ? Is this the way it's supposed to work ? If so, is there any documentation mentioning it ? I couldn't find anything on the subject on MSDN or Google...

And more importantly, is there a workaround ? (other than explicitly calling LoadProperty from the worker thread...)

Any help would be very appreciated

PS: I'm using VS2010, so it's EF 4.0. I don't know if it was the same in the previous version of EF...

解决方案

Is this by design? Yes; any call to Load, implicit or explicit, will eventually go through the ObjectContext, and ObjectContext is documented to be not thread-safe.

A possible workaround would be to detach the entity from the object context in the worker thread and attach it to an object context in the current thread.

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