.NET 委派深度解析（下）：非同步模式、呼叫清單、異步回調實戰

前言

承接上集委派基礎與事件封裝，本文深入探討多播委派呼叫清單、非同步委派模式 (APM)、輪詢 vs 回調三種異步處理方式，並以「工人 Peter、老闆 Boss、宇宙 Universe」故事串聯實戰場景。

1. 取得所有呼叫結果：GetInvocationList()

多播委派內部維護呼叫清單，預設同步串列呼叫，僅回傳最後一個結果。若需每個訂閱者回傳值：

public void DoWork()
{
    Console.WriteLine("Worker: work completed");
    if (completed != null)
    {
        foreach (WorkCompleted wc in completed.GetInvocationList())
        {
            int grade = wc(); // 同步逐一呼叫
            Console.WriteLine($"Worker grade = {grade}");
        }
    }
}

輸出

Worker: work completed
Better...          // Boss 回傳 4
Worker grade = 4
Universe pleased... // Universe 回傳 7
Worker grade = 7

2. 異步觸發並忽略結果：BeginInvoke 無回調

訂閱者處理耗時長 (Thread.Sleep)，Peter 不想等待：

public void DoWork()
{
    Console.WriteLine("Worker: work completed");
    if (completed != null)
    {
        foreach (WorkCompleted wc in completed.GetInvocationList())
        {
            wc.BeginInvoke(null, null); // 火後不理，執行緒池處理
        }
    }
}

輸出順序變化

Worker: work completed
Main: worker completed work  // Peter 立即繼續
Better...                     // 稍後在背景執行緒輸出
Universe pleased...

缺點：無法取得回傳值、無法捕捉例外、無法知道完成時機。

3. 異步觸發 + 輪詢等待：BeginInvoke + IsCompleted

Peter 想要結果，但不想阻塞主執行緒太久：

public void DoWork()
{
    Console.WriteLine("Worker: work completed");
    if (completed != null)
    {
        foreach (WorkCompleted wc in completed.GetInvocationList())
        {
            IAsyncResult ar = wc.BeginInvoke(null, null);
            while (!ar.IsCompleted) Thread.Sleep(1); // 忙碌等待 (不建議生產環境)
            int grade = wc.EndInvoke(ar);
            Console.WriteLine($"Worker grade = {grade}");
        }
    }
}

輸出

Worker: work completed
Better...
Worker grade = 6
Universe pleased...
Worker grade = 7
Main: worker completed work

缺點：輪詢浪費 CPU，同步等待喪失異步優勢。

4. 異步回調模式：BeginInvoke + AsyncCallback (推薦)

Peter 註冊回調，完成時自動通知：

public void DoWork()
{
    Console.WriteLine("Worker: work completed");
    if (completed != null)
    {
        foreach (WorkCompleted wc in completed.GetInvocationList())
        {
            wc.BeginInvoke(WorkGraded, wc); // 傳遞委派本身作為狀態
        }
    }
}

private void WorkGraded(IAsyncResult ar)
{
    WorkCompleted wc = (WorkCompleted)ar.AsyncState;
    int grade = wc.EndInvoke(ar); // 必須呼叫 EndInvoke 取得結果/傳播例外
    Console.WriteLine($"Worker grade = {grade}");
}

輸出 (非同步完成順序)

Worker: work completed
Main: worker completed work  // 立即返回
Better...
Worker grade = 6
Universe pleased...
Worker grade = 7

完整程式碼架構

delegate void WorkStarted();
delegate void WorkProgressing();
delegate int WorkCompleted(); // 有回傳值

class Worker
{
    public event WorkStarted Started;
    public event WorkProgressing Progressing;
    public event WorkCompleted Completed;

    public void DoWork()
    {
        Console.WriteLine("Worker: work started");
        Started?.Invoke();
        Console.WriteLine("Worker: work progressing");
        Progressing?.Invoke();
        Console.WriteLine("Worker: work completed");
        // 此處插入四種模式任一實作
    }

    // 回調模式專用
    private void WorkGraded(IAsyncResult ar) { ... }
}

class Boss
{
    public int WorkCompleted()
    {
        Thread.Sleep(3000); // 模擬耗時評估
        Console.WriteLine("Better...");
        return 6;
    }
}

class Universe
{
    public static void WorkerStartedWork() => Console.WriteLine("Universe notices worker starting work");
    public static int WorkerCompletedWork()
    {
        Thread.Sleep(4000);
        Console.WriteLine("Universe pleased with worker's work");
        return 7;
    }
}

class Program
{
    static void Main()
    {
        var peter = new Worker();
        var boss = new Boss();
        peter.Completed += boss.WorkCompleted;
        peter.Started += Universe.WorkerStartedWork;
        peter.Completed += Universe.WorkerCompletedWork;
        peter.DoWork();
        Console.WriteLine("Main: worker completed work");
        Console.ReadLine();
    }
}

APM 模式關鍵規則

1. 每個 BeginInvoke 必須配對 EndInvoke，否則洩漏資源且例外被吞沒
2. AsyncState 傳遞委派本身，以便回調中呼叫 EndInvoke
3. 回調執行於 ThreadPool 執行緒，UI 更新需 Control.Invoke / Dispatcher.Invoke
4. 例外傳播：EndInvoke 會重新擲回委派中未處理的例外

現代替代方案：Task-based Asynchronous Pattern (TAP)

// .NET 4.5+ 建議使用 Task/await
public async Task DoWorkAsync()
{
    Console.WriteLine("Worker: work completed");
    if (completed != null)
    {
        var tasks = completed.GetInvocationList()
            .Cast()
            .Select(wc => Task.Factory.FromAsync(wc.BeginInvoke, wc.EndInvoke, null));
        var grades = await Task.WhenAll(tasks);
        foreach (var g in grades) Console.WriteLine($"Worker grade = {g}");
    }
}

優點：可讀性高、例外處理自然、支援取消、進度報告、組合子。

總結

理解委派非同步模式是掌握 .NET 非同步程式設計基石，雖然現代開發多用 async/await，但 APM 仍廣泛存在於舊版 API 與底層庫中，深入理解有助於除錯與維護。