【你不必定知曉的】C#取消異步操做

【你不必定知曉的】C#取消異步操做

在.Net和C#中運行異步代碼至關簡單，由於咱們有時候須要取消正在進行的異步操做，經過本文，能夠掌握 經過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做。

可使用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  } }

 已經再也不推薦這種方式來完成異步和長時間運行的操做，可是大部分概念在如今依舊可使用。

 Task橫空出世

Task表明一個異步操做， 該類表示一個異步不返回值的操做， 泛型版本Task<TResult>表示異步有返回值的操做。

可以使用async/await 語法糖代碼去完成異步操做。

 

如下建立一個簡單的長時間運行的操做：

/// <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; }); } // 這裏咱們使用Thread.Sleep 模仿長時間運行的操做

 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#取消異步操做分爲

 

 

① 讓代碼可取消（Cancellable）

由於一些緣由，長時間運行的操做花費了 冗長的時間（須要取消，避免佔用資源）； 或者不肯意再等待執行結果了

咱們會取消異步操做。

 

爲完成目的須要在 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; }

 

在長時間運行的操做中監測  IsCancellationRequested方法 (當前是否發生取消命令)，這裏我傾向去包裝一個 TaskCanceledException異常類（給上層方法調用者更多處理的可能性）； 固然能夠調用 ThrowIfCancellationRequested方法拋出 OperationCanceledException異常。

② 觸發取消命令

CancellationToken結構體至關於打入在異步操做內部的楔子，隨時等候後方發來的取消命令。

操縱以上CancellationToken狀態的對象是 CancellationTokenSource，這個對象是取消操做的命令發佈者。

 

默認的構造函數就支持了 超時取消：

// 如下代碼 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： 高階操做，完成手動取消：

天然咱們關注到 CancellationSource 的幾個方法， 要想在異步操做的時候 手動取消操做，須要創建另外的線程 等待手動取消操做的指令。

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; } }
// 以上是一個控制檯程序，異步接收控制檯輸入，發出取消命令。

 

附②：高階操做，取消 non-Cancellable任務 ：

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

LongRunningCancellableOperation(int loop, CancellationToken cancellationToken) 並不提供參數2的傳入，至關於不容許 打入楔子。

 

這時咱們怎樣取消 這樣的non-Cancellable 任務？

 

可考慮利用 Task.WhenAny( params tasks) 操做曲線取消：

• 利用TaskCompletionSource 註冊異步可取消任務
• 等待待non-cancellable 操做和以上創建的 異步取消操做

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; }

像上面代碼同樣執行取消命令 ：

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; } }

  總結：

大多數狀況下，咱們不須要編寫自定義可取消任務，由於咱們只須要使用現有API。但要知道它是如何在幕後工做老是好的。

 

參考資料： 

https://johnthiriet.com/cancel-asynchronous-operation-in-csharp/

https://stackoverflow.com/questions/4238345/asynchronously-wait-for-taskt-to-complete-with-timeout

https://github.com/App-vNext/Polly/wiki/Timeout

 

源做者： Julian_醬