【你不一定知曉的】C#取消異步操作

在.Net和C#中運行異步代碼相當簡單，因為我們有時候需要取消正在進行的異步操作，通過本文，可以掌握通過CancellationToken取消任務（包括non-cancellable任務）。

早期

早期.Net 使用 BackgroundWorker 完成異步長時間運行操作。

可以使用CacnelAsync方法設置 CancellationPending = true

private void BackgroundLongRunningTask(object sender, DoWorkEventArgs e) { BackgroundWorker worker = (BackgroundWorker)sender; for (int i = 1; i <= 10000; i++) { if (worker.CancellationPending == true) { e.Cancel = true; break; } // Do something  } }

已經不再推薦這種方式來完成異步和長時間運行的操作，但是大部分概念在現在依舊可以使用。

Task橫空出世

Task代表一個異步操作， 該類表示一個異步不返回值的操作， 泛型版本Task<TResult>表示異步有返回值的操作。

可使用async/await 語法糖代碼去完成異步操作。

以下創建一個簡單的長時間運行的操作：

/// <summary> /// Compute a value for a long time. /// </summary> /// <returns>The value computed.</returns> /// <param name="loop">Number of iterations to do.</param> private static Task<decimal> LongRunningOperation(int loop) { // Start a task and return it return Task.Run(() => { decimal result = 0; // Loop for a defined number of iterations for (int i = 0; i < loop; i++) { // Do something that takes times like a Thread.Sleep in .NET Core 2. Thread.Sleep(10); result += i; } return result; }); } // 這里我們使用Thread.Sleep 模仿長時間運行的操作

簡單異步調用代碼：

public static async Task ExecuteTaskAsync() { Console.WriteLine(nameof(ExecuteTaskAsync)); Console.WriteLine("Result {0}", await LongRunningOperation(100)); Console.WriteLine("Press enter to continue"); Console.ReadLine(); }

敲黑板： C#取消異步操作分為

① 讓代碼可取消（Cancellable）

因為一些原因，長時間運行的操作花費了冗長的時間（需要取消，避免占用資源）；或者不願意再等待執行結果了

我們會取消異步操作。

為完成目的需要在長時間運行的異步任務中傳入CancellationToken:

/// <summary> /// Compute a value for a long time. /// </summary> /// <returns>The value computed.</returns> /// <param name="loop">Number of iterations to do.</param> /// <param name="cancellationToken">The cancellation token.</param> private static Task<decimal> LongRunningCancellableOperation(int loop, CancellationToken cancellationToken) { Task<decimal> task = null; // Start a task and return it task = Task.Run(() => { decimal result = 0; // Loop for a defined number of iterations for (int i = 0; i < loop; i++) { // Check if a cancellation is requested, if yes, // throw a TaskCanceledException. if (cancellationToken.IsCancellationRequested) throw new TaskCanceledException(task); // Do something that takes times like a Thread.Sleep in .NET Core 2. Thread.Sleep(10); result += i; } return result; }); return task; }

在長時間運行的操作中監測 IsCancellationRequested方法 (當前是否發生取消命令)，這里我傾向去包裝一個 TaskCanceledException異常類（給上層方法調用者更多處理的可能性）；當然可以調用 ThrowIfCancellationRequested方法拋出 OperationCanceledException異常。

② 觸發取消命令

CancellationToken結構體相當於打入在異步操作內部的楔子，隨時等候后方發來的取消命令。

操縱以上CancellationToken狀態的對象是 CancellationTokenSource，這個對象是取消操作的命令發布者。

默認的構造函數就支持了超時取消：

// 以下代碼 利用 CancellationSource默認構造函數 完成超時取消 public static async Task ExecuteTaskWithTimeoutAsync(TimeSpan timeSpan) { Console.WriteLine(nameof(ExecuteTaskWithTimeoutAsync)); using (var cancellationTokenSource = new CancellationTokenSource(timeSpan)) { try { var result = await LongRunningCancellableOperation(500, cancellationTokenSource.Token); Console.WriteLine("Result {0}", result); } catch (TaskCanceledException) { Console.WriteLine("Task was cancelled"); } } Console.WriteLine("Press enter to continue"); Console.ReadLine(); }

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附①：高階操作，完成手動取消：

自然我們關注到 CancellationSource 的幾個方法，要想在異步操作的時候手動取消操作，需要建立另外的線程等待手動取消操作的指令。

public static async Task ExecuteManuallyCancellableTaskAsync() { Console.WriteLine(nameof(ExecuteManuallyCancellableTaskAsync)); using (var cancellationTokenSource = new CancellationTokenSource()) { // Creating a task to listen to keyboard key press var keyBoardTask = Task.Run(() => { Console.WriteLine("Press enter to cancel"); Console.ReadKey(); // Cancel the task  cancellationTokenSource.Cancel(); }); try { var longRunningTask = LongRunningCancellableOperation(500, cancellationTokenSource.Token); var result = await longRunningTask; Console.WriteLine("Result {0}", result); Console.WriteLine("Press enter to continue"); } catch (TaskCanceledException) { Console.WriteLine("Task was cancelled"); } await keyBoardTask; } }
// 以上是一個控制台程序，異步接收控制台輸入，發出取消命令。

附②：高階操作，取消 non-Cancellable任務：

有時候，異步操作代碼並不提供對 Cancellation的支持，也就是以上長時間運行的異步操作

LongRunningCancellableOperation(int loop, CancellationToken cancellationToken) 並不提供參數2的傳入，相當於不允許打入楔子。

這時我們怎樣取消這樣的non-Cancellable 任務？

可考慮利用 Task.WhenAny( params tasks) 操作曲線取消：

利用TaskCompletionSource 注冊異步可取消任務
等待待non-cancellable 操作和以上建立的異步取消操作

private static async Task<decimal> LongRunningOperationWithCancellationTokenAsync(int loop, CancellationToken cancellationToken) { // We create a TaskCompletionSource of decimal var taskCompletionSource = new TaskCompletionSource<decimal>(); // Registering a lambda into the cancellationToken cancellationToken.Register(() => { // We received a cancellation message, cancel the TaskCompletionSource.Task  taskCompletionSource.TrySetCanceled(); }); var task = LongRunningOperation(loop); // Wait for the first task to finish among the two var completedTask = await Task.WhenAny(task, taskCompletionSource.Task); return await completedTask; }

像上面代碼一樣執行取消命令：

public static async Task CancelANonCancellableTaskAsync() { Console.WriteLine(nameof(CancelANonCancellableTaskAsync)); using (var cancellationTokenSource = new CancellationTokenSource()) { // Listening to key press to cancel var keyBoardTask = Task.Run(() => { Console.WriteLine("Press enter to cancel"); Console.ReadKey(); // Sending the cancellation message  cancellationTokenSource.Cancel(); }); try { // Running the long running task var longRunningTask = LongRunningOperationWithCancellationTokenAsync(100, cancellationTokenSource.Token); var result = await longRunningTask; Console.WriteLine("Result {0}", result); Console.WriteLine("Press enter to continue"); } catch (TaskCanceledException) { Console.WriteLine("Task was cancelled"); } await keyBoardTask; } }