为什么 C# 事件处理程序等于 MSN 示例中的事件

以下是MSN网站上的一个示例。 这是一个很好的例子，我只是不明白这行是什么：

EventHandler<CustomEventArgs> handler = RaiseCustomEvent;

在做什么？

1. RaiseCustomEvent不是基于程序顶部定义的事件吗？

2. 为什么事件等同于事件处理程序？ 这是两种不同的类型。

3. 在哪里初始化RaiseCustomEvent？如果它没有初始化，我们如何复制它，或者为什么我们要将未初始化的东西复制到其他东西上？

4. 那里的handler变量有什么用？ 这是事件还是事件处理程序？

我很困惑，正在努力理解这个事件/事件处理程序/委托问题。

下面是来自 MSN 的示例代码

namespace DotNetEvents
{
using System;
using System.Collections.Generic;
// Define a class to hold custom event info 
public class CustomEventArgs : EventArgs
{
    public CustomEventArgs(string s)
    {
        message = s;
    }
    private string message;
    public string Message
    {
        get { return message; }
        set { message = value; }
    }
}
// Class that publishes an event 
class Publisher
{
    // Declare the event using EventHandler<T> 
    public event EventHandler<CustomEventArgs> RaiseCustomEvent;
    public void DoSomething()
    {
        // Write some code that does something useful here 
        // then raise the event. You can also raise an event 
        // before you execute a block of code.
        OnRaiseCustomEvent(new CustomEventArgs("Did something"));
    }
    // Wrap event invocations inside a protected virtual method 
    // to allow derived classes to override the event invocation behavior 
    protected virtual void OnRaiseCustomEvent(CustomEventArgs e)
    {
        // Make a temporary copy of the event to avoid possibility of 
        // a race condition if the last subscriber unsubscribes 
        // immediately after the null check and before the event is raised.
        EventHandler<CustomEventArgs> handler = RaiseCustomEvent;
        // Event will be null if there are no subscribers 
        if (handler != null)
        {
            // Format the string to send inside the CustomEventArgs parameter
            e.Message += String.Format(" at {0}", DateTime.Now.ToString());
            // Use the () operator to raise the event.
            handler(this, e);
        }
    }
}
//Class that subscribes to an event 
class Subscriber
{
    private string id;
    public Subscriber(string ID, Publisher pub)
    {
        id = ID;
        // Subscribe to the event using C# 2.0 syntax
        pub.RaiseCustomEvent += HandleCustomEvent;
    }
    // Define what actions to take when the event is raised. 
    void HandleCustomEvent(object sender, CustomEventArgs e)
    {
        Console.WriteLine(id + " received this message: {0}", e.Message);
    }
}
class Program
{
    static void Main(string[] args)
    {
        Publisher pub = new Publisher();
        Subscriber sub1 = new Subscriber("sub1", pub);
        Subscriber sub2 = new Subscriber("sub2", pub);
        // Call the method that raises the event.
        pub.DoSomething();
        // Keep the console window open
        Console.WriteLine("Press Enter to close this window.");
        Console.ReadLine();
    }
}
}