C# 事件如何在幕后工作? [英] How do C# Events work behind the scenes?

问题描述

我正在使用 C#、.NET 3.5.我了解如何利用事件，如何在我的班级中声明它们，如何从其他地方挂钩它们等等.一个人为的例子:

I'm using C#, .NET 3.5. I understand how to utilize events, how to declare them in my class, how to hook them from somewhere else, etc. A contrived example:

public class MyList
{
    private List<string> m_Strings = new List<string>();
    public EventHandler<EventArgs> ElementAddedEvent;

    public void Add(string value)
    {
        m_Strings.Add(value);
        if (ElementAddedEvent != null)
            ElementAddedEvent(value, EventArgs.Empty);
    }
}

[TestClass]
public class TestMyList
{
    private bool m_Fired = false;

    [TestMethod]
    public void TestEvents()
    {
        MyList tmp = new MyList();
        tmp.ElementAddedEvent += new EventHandler<EventArgs>(Fired);
        tmp.Add("test");
        Assert.IsTrue(m_Fired);
    }

    private void Fired(object sender, EventArgs args)
    {
        m_Fired = true;
    }
}

然而，我不理解的是，当一个人声明一个事件处理程序时

However, what I do not understand, is when one declares an event handler

public EventHandler<EventArgs> ElementAddedEvent;

它从未被初始化 - 那么，ElementAddedEvent 究竟是什么?它指向什么?以下将不起作用，因为 EventHandler 从未初始化:

It's never initialized - so what, exactly, is ElementAddedEvent? What does it point to? The following won't work, because the EventHandler is never initialized:

[TestClass]
public class TestMyList
{
    private bool m_Fired = false;

    [TestMethod]
    public void TestEvents()
    {
        EventHandler<EventArgs> somethingHappend;
        somethingHappend += new EventHandler<EventArgs>(Fired);
        somethingHappend(this, EventArgs.Empty);
        Assert.IsTrue(m_Fired);
    }

    private void Fired(object sender, EventArgs args)
    {
        m_Fired = true;
    }
}

我注意到有一个 EventHandler.CreateDelegate(...)，但所有方法签名都表明这仅用于通过典型的 ElementAddedEvent += new EventHandler(MyMethod) 将 Delegates 附加到已经存在的 EventHandler.

I notice that there is an EventHandler.CreateDelegate(...), but all the method signatures suggest this is only used for attaching Delegates to an already existing EventHandler through the typical ElementAddedEvent += new EventHandler(MyMethod).

我不确定我正在尝试做的什么是否会有所帮助......但最终我想在 LINQ 中提出一个抽象的父 DataContext，它的孩子可以注册哪些表类型他们想要观察"，所以我可以有诸如更新前和更新后之类的事件，但特定于类型.像这样的:

I'm not sure if what I am trying to do will help... but ultimately I'd like to come up with an abstract parent DataContext in LINQ whose children can register which table Types they want "observed" so I can have events such as BeforeUpdate and AfterUpdate, but specific to types. Something like this:

public class BaseDataContext : DataContext
{
    private static Dictionary<Type, Dictionary<ChangeAction, EventHandler>> m_ObservedTypes = new Dictionary<Type, Dictionary<ChangeAction, EventHandler>>();

    public static void Observe(Type type)
    {
        if (m_ObservedTypes.ContainsKey(type) == false)
        {
            m_ObservedTypes.Add(type, new Dictionary<ChangeAction, EventHandler>());

            EventHandler eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler;
            m_ObservedTypes[type].Add(ChangeAction.Insert, eventHandler);

            eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler;
            m_ObservedTypes[type].Add(ChangeAction.Update, eventHandler);

            eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler;
            m_ObservedTypes[type].Add(ChangeAction.Delete, eventHandler);
        }
    }

    public static Dictionary<Type, Dictionary<ChangeAction, EventHandler>> Events
    {
        get { return m_ObservedTypes; }
    }
}


public class MyClass
{
    public MyClass()
    {
        BaseDataContext.Events[typeof(User)][ChangeAction.Update] += new EventHandler(OnUserUpdate);
    }

    public void OnUserUpdated(object sender, EventArgs args)
    {
        // do something
    }
}

考虑到这一点让我意识到我并不真正了解事件背后发生的事情 - 我想了解:)

Thinking about this made me realize I don't really understand what's happening under the hod with events - and I would like to understand :)

推荐答案

我已经在 一篇文章，但这里是摘要，假设您对 代表 自己:

I've written this up in a fair amount of detail in an article, but here's the summary, assuming you're reasonably happy with delegates themselves:

• 事件只是添加"方法和删除"方法，就像属性实际上只是获取"方法和设置"方法一样.(事实上​​，CLI 也允许使用raise/fire"方法，但 C# 从不生成此方法.)元数据通过对方法的引用来描述事件.
• 当您声明一个类字段事件(就像您的 ElementAddedEvent)编译器生成方法和一个私有字段(与委托的类型相同).在类中，当您引用 ElementAddedEvent 时，您指的是该字段.在课堂之外，您指的是该领域.
• 当任何人(使用 += 运算符)订阅调用 add 方法的事件时.当他们取消订阅(使用 -= 运算符)调用删除时.

• 对于类似字段的事件，有一些同步，否则添加/删除只需调用 Delegate.合并/删除以更改自动生成字段的值.这两个操作都分配给支持字段 - 请记住，委托是不可变的.换句话说，自动生成的代码很像这样:

• An event is just an "add" method and a "remove" method, in the same way that a property is really just a "get" method and a "set" method. (In fact, the CLI allows a "raise/fire" method as well, but C# never generates this.) Metadata describes the event with references to the methods.
• When you declare a field-like event (like your ElementAddedEvent) the compiler generates the methods and a private field (of the same type as the delegate). Within the class, when you refer to ElementAddedEvent you're referring to the field. Outside the class, you're referring to the field.
• When anyone subscribes to an event (with the += operator) that calls the add method. When they unsubscribe (with the -= operator) that calls the remove.

• For field-like events, there's some synchronization but otherwise the add/remove just call Delegate.Combine/Remove to change the value of the auto-generated field. Both of these operations assign to the backing field - remember that delegates are immutable. In other words, the autogenerated code is very much like this:

// Backing field
// The underscores just make it simpler to see what's going on here.
// In the rest of your source code for this class, if you refer to
// ElementAddedEvent, you're really referring to this field.
private EventHandler<EventArgs> __ElementAddedEvent;

// Actual event
public EventHandler<EventArgs> ElementAddedEvent
{
    add
    {
        lock(this)
        {
            // Equivalent to __ElementAddedEvent += value;
            __ElementAddedEvent = Delegate.Combine(__ElementAddedEvent, value);
        }
    }
    remove
    {
        lock(this)
        {
            // Equivalent to __ElementAddedEvent -= value;
            __ElementAddedEvent = Delegate.Remove(__ElementAddedEvent, value);
        }
    }
}

在您的情况下，生成字段的初始值为 null - 如果删除所有订阅者，它将始终再次变为 null，因为那是Delegate.Remove 的行为.

The initial value of the generated field in your case is null - and it will always become null again if all subscribers are removed, as that is the behaviour of Delegate.Remove.

如果你想要一个no-op"处理程序来订阅你的事件，以避免无效性检查，你可以这样做:

If you want a "no-op" handler to subscribe to your event, so as to avoid the nullity check, you can do:

public EventHandler<EventArgs> ElementAddedEvent = delegate {};

delegate {} 只是一个匿名方法，它不关心它的参数并且什么都不做.

The delegate {} is just an anonymous method which doesn't care about its parameters and does nothing.

如果还有什么不清楚的地方，请询问，我会尽力帮助的！

If there's anything that's still unclear, please ask and I'll try to help!

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