C#的事件(Event)是什么？如何发布和订阅事件？

public class DownloadManager
{
    // 使用内置泛型委托定义事件
    public event EventHandler<DownloadEventArgs> DownloadCompleted;
<pre class='brush:php;toolbar:false;'>// 自定义事件参数类
public class DownloadEventArgs : EventArgs
{
    public string FileName { get; set; }
    public bool Success { get; set; }
}

// 触发事件的方法
protected virtual void OnDownloadCompleted(bool success, string fileName)
{
    DownloadCompleted?.Invoke(this, new DownloadEventArgs 
    { 
        Success = success, 
        FileName = fileName 
    });
}

// 模拟下载完成并发布事件
public void SimulateDownload()
{
    Console.WriteLine("开始下载...");
    // 模拟操作
    Thread.Sleep(1000);
    OnDownloadCompleted(true, "example.zip");
}

class Program
{
    static void Main()
    {
        var downloader = new DownloadManager();
<pre class='brush:php;toolbar:false;'>    // 订阅事件
    downloader.DownloadCompleted += OnDownloadFinished;

    // 启动下载（会触发事件）
    downloader.SimulateDownload();
}

// 事件处理方法
static void OnDownloadFinished(object sender, DownloadManager.DownloadEventArgs e)
{
    if (e.Success)
    {
        Console.WriteLine($"文件 {e.FileName} 下载成功！");
    }
    else
    {
        Console.WriteLine("下载失败。");
    }
}

downloader.DownloadCompleted += (s, e) => Console.WriteLine($"下载完成: {e.FileName}");

downloader.DownloadCompleted -= OnDownloadFinished;