为了理解它们的区别，你可以看看这两个例子

委托的示例(在本例中是Action -这是一种不返回值的委托)

public class Animal
{
    public Action Run {get; set;}

    public void RaiseEvent()
    {
        if (Run != null)
        {
            Run();
        }
    }
}

要使用委托，你应该这样做:

Animal animal= new Animal();
animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running") ;
animal.RaiseEvent();

这段代码运行良好，但可能有一些弱点。

例如，如果我这样写:

animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running");
animal.Run = () => Console.WriteLine("I'm sleeping") ;

在最后一行代码中，我重写了前面的行为，只是缺少了一个+(我已经使用=而不是+=)

另一个弱点是每个使用Animal类的类都可以直接调用委托。例如，Animal . run()或Animal . run . invoke()在Animal类之外有效。

为了避免这些弱点，你可以在c#中使用事件。

你的动物职业会这样改变:

public class ArgsSpecial : EventArgs
{
    public ArgsSpecial (string val)
    {
        Operation=val;
    }

    public string Operation {get; set;}
} 

public class Animal
{
    // Empty delegate. In this way you are sure that value is always != null 
    // because no one outside of the class can change it.
    public event EventHandler<ArgsSpecial> Run = delegate{} 
        
    public void RaiseEvent()
    {  
         Run(this, new ArgsSpecial("Run faster"));
    }
}

调用事件

 Animal animal= new Animal();
 animal.Run += (sender, e) => Console.WriteLine("I'm running. My value is {0}", e.Operation);
 animal.RaiseEvent();

差异:

You aren't using a public property but a public field (using events, the compiler protects your fields from unwanted access) Events can't be assigned directly. In this case, it won't give rise to the previous error that I have showed with overriding the behavior. No one outside of your class can raise or invoke the event. For example, animal.Run() or animal.Run.Invoke() are invalid outside the Animal class and will produce compiler errors. Events can be included in an interface declaration, whereas a field cannot

注:

EventHandler被声明为如下委托:

public delegate void EventHandler (object sender, EventArgs e)

它接受一个发送者(Object类型)和事件参数。如果来自静态方法，则发送者为空。

这个例子使用了EventHandler<ArgsSpecial>，也可以使用EventHandler来代替。

请参阅这里有关EventHandler的文档

注意:如果你有c# 5.0 Unleashed，请阅读第18章“事件”中的“委托的普通使用限制”，以更好地理解两者之间的区别。

---

举一个简单而具体的例子总是对我有帮助。这是给社区的。首先，我将展示如何单独使用委托来完成事件为我们做的事情。然后，我将展示相同的解决方案如何使用EventHandler的实例。然后我解释为什么我们不想做我在第一个例子中解释的事情。这篇文章的灵感来自John Skeet的一篇文章。

例1:使用公共委托

假设我有一个只有一个下拉框的WinForms应用程序。下拉列表绑定到List<Person>。其中Person具有Id，名称，昵称，头发颜色的属性。在主窗体上有一个自定义用户控件，显示此人的属性。当某人在下拉菜单中选择某人时，用户控件中的标签将更新以显示所选人员的属性。

下面是它的工作原理。我们有三个文件可以帮助我们把这些放在一起:

cs——静态类保存委托 Form1.cs——主要形式 cs——用户控件显示所有细节

下面是每个类的相关代码:

class Mediator
{
    public delegate void PersonChangedDelegate(Person p); //delegate type definition
    public static PersonChangedDelegate PersonChangedDel; //delegate instance. Detail view will "subscribe" to this.
    public static void OnPersonChanged(Person p) //Form1 will call this when the drop-down changes.
    {
        if (PersonChangedDel != null)
        {
            PersonChangedDel(p);
        }
    }
}

这是我们的用户控件:

public partial class DetailView : UserControl
{
    public DetailView()
    {
        InitializeComponent();
        Mediator.PersonChangedDel += DetailView_PersonChanged;
    }

    void DetailView_PersonChanged(Person p)
    {
        BindData(p);
    }

    public void BindData(Person p)
    {
        lblPersonHairColor.Text = p.HairColor;
        lblPersonId.Text = p.IdPerson.ToString();
        lblPersonName.Text = p.Name;
        lblPersonNickName.Text = p.NickName;

    }
}

最后，我们在Form1.cs中有以下代码。这里我们正在调用OnPersonChanged，它调用任何订阅到委托的代码。

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    Mediator.OnPersonChanged((Person)comboBox1.SelectedItem); //Call the mediator's OnPersonChanged method. This will in turn call all the methods assigned (i.e. subscribed to) to the delegate -- in this case `DetailView_PersonChanged`.
}

好的。这就是不使用事件只使用委托就能让它工作的方法。我们只是把一个公共委托放到一个类中——你可以让它是静态的，或者是单例的，等等。太好了。

但是，但是，但是，我们不想做我上面描述的事情。因为公共字段有很多不好的原因。那么我们有什么选择呢?正如约翰·斯基特所描述的，我们有以下几种选择:

A public delegate variable (this is what we just did above. don't do this. i just told you above why it's bad) Put the delegate into a property with a get/set (problem here is that subscribers could override each other -- so we could subscribe a bunch of methods to the delegate and then we could accidentally say PersonChangedDel = null, wiping out all of the other subscriptions. The other problem that remains here is that since the users have access to the delegate, they can invoke the targets in the invocation list -- we don't want external users having access to when to raise our events. A delegate variable with AddXXXHandler and RemoveXXXHandler methods

第三个选项本质上是事件给我们的。当我们声明一个EventHandler时，它给了我们对委托的访问，不是公开的，不是作为属性，而是作为我们称之为事件的东西它有添加/删除访问器。

让我们看看同样的程序是什么样子，但是现在使用Event而不是public委托(我也将我们的Mediator更改为单例):

例2:使用EventHandler代替公共委托

中介:

class Mediator
{

    private static readonly Mediator _Instance = new Mediator();

    private Mediator() { }

    public static Mediator GetInstance()
    {
        return _Instance;
    }

    public event EventHandler<PersonChangedEventArgs> PersonChanged; //this is just a property we expose to add items to the delegate.

    public void OnPersonChanged(object sender, Person p)
    {
        var personChangedDelegate = PersonChanged as EventHandler<PersonChangedEventArgs>;
        if (personChangedDelegate != null)
        {
            personChangedDelegate(sender, new PersonChangedEventArgs() { Person = p });
        }
    }
}

注意，如果你在EventHandler上F12，它会显示你的定义只是一个泛型的委托，带有额外的"sender"对象:

public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);

用户控件:

public partial class DetailView : UserControl
{
    public DetailView()
    {
        InitializeComponent();
        Mediator.GetInstance().PersonChanged += DetailView_PersonChanged;
    }

    void DetailView_PersonChanged(object sender, PersonChangedEventArgs e)
    {
        BindData(e.Person);
    }

    public void BindData(Person p)
    {
        lblPersonHairColor.Text = p.HairColor;
        lblPersonId.Text = p.IdPerson.ToString();
        lblPersonName.Text = p.Name;
        lblPersonNickName.Text = p.NickName;

    }
}

最后，这是Form1.cs的代码:

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
        Mediator.GetInstance().OnPersonChanged(this, (Person)comboBox1.SelectedItem);
}

因为EventHandler想要和EventArgs作为参数，我创建这个类只有一个属性:

class PersonChangedEventArgs
{
    public Person Person { get; set; }
}

希望这向您展示了为什么我们有事件，以及它们作为委托如何不同——但功能相同。

An event in .net is a designated combination of an Add method and a Remove method, both of which expect some particular type of delegate. Both C# and vb.net can auto-generate code for the add and remove methods which will define a delegate to hold the event subscriptions, and add/remove the passed in delegagte to/from that subscription delegate. VB.net will also auto-generate code (with the RaiseEvent statement) to invoke the subscription list if and only if it is non-empty; for some reason, C# doesn't generate the latter.

Note that while it is common to manage event subscriptions using a multicast delegate, that is not the only means of doing so. From a public perspective, a would-be event subscriber needs to know how to let an object know it wants to receive events, but it does not need to know what mechanism the publisher will use to raise the events. Note also that while whoever defined the event data structure in .net apparently thought there should be a public means of raising them, neither C# nor vb.net makes use of that feature.

Delegate是一个类型安全的函数指针。事件是使用委托的发布者-订阅者设计模式的实现。

活动和代表之间的误解太大了!!委托指定TYPE(例如类或接口)，而事件只是一种成员(例如字段、属性等)。并且，就像任何其他类型的成员一样，事件也有类型。然而，在事件的情况下，事件的类型必须由委托指定。例如，不能声明由接口定义的类型的事件。

最后，我们可以做以下观察:事件的类型必须由委托定义。这是事件和委托之间的主要关系，在第二节中描述。18定义ECMA-335 (CLI)分区I到VI的事件:

在典型用法中，TypeSpec(如果存在)标识一个签名与传递给事件的fire方法的参数匹配的委托。

However, this fact does NOT imply that an event uses a backing delegate field. In truth, an event may use a backing field of any different data structure type of your choice. If you implement an event explicitly in C#, you are free to choose the way you store the event handlers (note that event handlers are instances of the type of the event, which in turn is mandatorily a delegate type---from the previous Observation). But, you can store those event handlers (which are delegate instances) in a data structure such as a List or a Dictionary or any other else, or even in a backing delegate field. But don’t forget that it is NOT mandatory that you use a delegate field.

除了语法和操作属性之外，还有语义上的差异。

从概念上讲，委托是函数模板;也就是说，它们表达了一个函数必须遵守的契约，以便被认为是委托的“类型”。

事件代表……嗯,事件。它们的目的是在发生事情时提醒某人，是的，它们遵循委托定义，但它们不是同一件事。

即使它们是完全相同的东西(在语法上和IL代码中)，仍然会存在语义差异。一般来说，我更喜欢对两个不同的概念使用两个不同的名称，即使它们以相同的方式实现(这并不意味着我喜欢使用两次相同的代码)。