用简单的方式定义about event:

事件是对具有两个限制的委托的引用

不能直接调用 不能直接赋值(例如eventObj = delegateMethod)

以上两点是委托的弱点，并在事件中得到解决。完整的代码示例，以显示fiddler的差异在这里https://dotnetfiddle.net/5iR3fB。

在Event和Delegate之间切换注释，以及调用/分配值给Delegate的客户端代码，以了解差异

下面是内联代码。

 /*
This is working program in Visual Studio.  It is not running in fiddler because of infinite loop in code.
This code demonstrates the difference between event and delegate
        Event is an delegate reference with two restrictions for increased protection

            1. Cannot be invoked directly
            2. Cannot assign value to delegate reference directly

Toggle between Event vs Delegate in the code by commenting/un commenting the relevant lines
*/

public class RoomTemperatureController
{
    private int _roomTemperature = 25;//Default/Starting room Temperature
    private bool _isAirConditionTurnedOn = false;//Default AC is Off
    private bool _isHeatTurnedOn = false;//Default Heat is Off
    private bool _tempSimulator = false;
    public  delegate void OnRoomTemperatureChange(int roomTemperature); //OnRoomTemperatureChange is a type of Delegate (Check next line for proof)
    // public  OnRoomTemperatureChange WhenRoomTemperatureChange;// { get; set; }//Exposing the delegate to outside world, cannot directly expose the delegate (line above), 
    public  event OnRoomTemperatureChange WhenRoomTemperatureChange;// { get; set; }//Exposing the delegate to outside world, cannot directly expose the delegate (line above), 

    public RoomTemperatureController()
    {
        WhenRoomTemperatureChange += InternalRoomTemperatuerHandler;
    }
    private void InternalRoomTemperatuerHandler(int roomTemp)
    {
        System.Console.WriteLine("Internal Room Temperature Handler - Mandatory to handle/ Should not be removed by external consumer of ths class: Note, if it is delegate this can be removed, if event cannot be removed");
    }

    //User cannot directly asign values to delegate (e.g. roomTempControllerObj.OnRoomTemperatureChange = delegateMethod (System will throw error)
    public bool TurnRoomTeperatureSimulator
    {
        set
        {
            _tempSimulator = value;
            if (value)
            {
                SimulateRoomTemperature(); //Turn on Simulator              
            }
        }
        get { return _tempSimulator; }
    }
    public void TurnAirCondition(bool val)
    {
        _isAirConditionTurnedOn = val;
        _isHeatTurnedOn = !val;//Binary switch If Heat is ON - AC will turned off automatically (binary)
        System.Console.WriteLine("Aircondition :" + _isAirConditionTurnedOn);
        System.Console.WriteLine("Heat :" + _isHeatTurnedOn);

    }
    public void TurnHeat(bool val)
    {
        _isHeatTurnedOn = val;
        _isAirConditionTurnedOn = !val;//Binary switch If Heat is ON - AC will turned off automatically (binary)
        System.Console.WriteLine("Aircondition :" + _isAirConditionTurnedOn);
        System.Console.WriteLine("Heat :" + _isHeatTurnedOn);

    }

    public async void SimulateRoomTemperature()
    {
        while (_tempSimulator)
        {
            if (_isAirConditionTurnedOn)
                _roomTemperature--;//Decrease Room Temperature if AC is turned On
            if (_isHeatTurnedOn)
                _roomTemperature++;//Decrease Room Temperature if AC is turned On
            System.Console.WriteLine("Temperature :" + _roomTemperature);
            if (WhenRoomTemperatureChange != null)
                WhenRoomTemperatureChange(_roomTemperature);
            System.Threading.Thread.Sleep(500);//Every second Temperature changes based on AC/Heat Status
        }
    }

}

public class MySweetHome
{
    RoomTemperatureController roomController = null;
    public MySweetHome()
    {
        roomController = new RoomTemperatureController();
        roomController.WhenRoomTemperatureChange += TurnHeatOrACBasedOnTemp;
        //roomController.WhenRoomTemperatureChange = null; //Setting NULL to delegate reference is possible where as for Event it is not possible.
        //roomController.WhenRoomTemperatureChange.DynamicInvoke();//Dynamic Invoke is possible for Delgate and not possible with Event
        roomController.SimulateRoomTemperature();
        System.Threading.Thread.Sleep(5000);
        roomController.TurnAirCondition (true);
        roomController.TurnRoomTeperatureSimulator = true;

    }
    public void TurnHeatOrACBasedOnTemp(int temp)
    {
        if (temp >= 30)
            roomController.TurnAirCondition(true);
        if (temp <= 15)
            roomController.TurnHeat(true);

    }
    public static void Main(string []args)
    {
        MySweetHome home = new MySweetHome();
    }


}

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.

对于生活在2020年的人们来说，想要一个干净的答案……

定义:

Delegate:定义一个函数指针。 事件:定义 (1)受保护接口，以及 (2)操作(+=，-=)，和 (3)优点:你不再需要使用new关键字。

关于形容词protected:

// eventTest.SomeoneSay = null;              // Compile Error.
// eventTest.SomeoneSay = new Say(SayHello); // Compile Error.

还要注意来自微软的这一部分:https://learn.microsoft.com/en-us/dotnet/standard/events/#raising-multiple-events

代码示例:

委托:

public class DelegateTest
{
    public delegate void Say(); // Define a pointer type "void <- ()" named "Say".
    private Say say;

    public DelegateTest() {
        say  = new Say(SayHello);     // Setup the field, Say say, first.
        say += new Say(SayGoodBye);
        
        say.Invoke();
    }
    
    public void SayHello() { /* display "Hello World!" to your GUI. */ }
    public void SayGoodBye() { /* display "Good bye!" to your GUI. */ }
}

事件:

public class EventTest
{
    public delegate void Say();
    public event Say SomeoneSay;  // Use the type "Say" to define event, an 
                                  // auto-setup-everything-good field for you.
    public EventTest() {
         SomeoneSay += SayHello;
         SomeoneSay += SayGoodBye;

         SomeoneSay();
    }
    
    public void SayHello() { /* display "Hello World!" to your GUI. */ }
    public void SayGoodBye() { /* display "Good bye!" to your GUI. */ }
}

参考:

事件和委托——解释c#中事件和委托模式之间的重要区别，以及它们为什么有用。: https://dzone.com/articles/event-vs-delegate

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

这是另一个很好的参考链接。 http://csharpindepth.com/Articles/Chapter2/Events.aspx

简单地说，本文的要点是——事件是对委托的封装。

引用自文章:

Suppose events didn't exist as a concept in C#/.NET. How would another class subscribe to an event? Three options: A public delegate variable A delegate variable backed by a property A delegate variable with AddXXXHandler and RemoveXXXHandler methods Option 1 is clearly horrible, for all the normal reasons we abhor public variables. Option 2 is slightly better, but allows subscribers to effectively override each other - it would be all too easy to write someInstance.MyEvent = eventHandler; which would replace any existing event handlers rather than adding a new one. In addition, you still need to write the properties. Option 3 is basically what events give you, but with a guaranteed convention (generated by the compiler and backed by extra flags in the IL) and a "free" implementation if you're happy with the semantics that field-like events give you. Subscribing to and unsubscribing from events is encapsulated without allowing arbitrary access to the list of event handlers, and languages can make things simpler by providing syntax for both declaration and subscription.

Event声明在委托实例上添加了一层抽象和保护。此保护可防止委托的客户端重置委托及其调用列表，并且仅允许从调用列表中添加或删除目标。