【你不一定知晓的】C#取消异步操作

在.Net和C#中运行异步代码相当简单，因为我们有时候需要取消正在进行的异步操作，通过本文，可以掌握通过CancellationToken取消任务（包括non-cancellable任务）。

早期

早期.Net 使用 BackgroundWorker 完成异步长时间运行操作。

可以使用CacnelAsync方法设置 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  } }

已经不再推荐这种方式来完成异步和长时间运行的操作，但是大部分概念在现在依旧可以使用。

Task横空出世

Task代表一个异步操作， 该类表示一个异步不返回值的操作， 泛型版本Task<TResult>表示异步有返回值的操作。

可使用async/await 语法糖代码去完成异步操作。

以下创建一个简单的长时间运行的操作：

/// <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; }); } // 这里我们使用Thread.Sleep 模仿长时间运行的操作

简单异步调用代码：

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#取消异步操作分为

① 让代码可取消（Cancellable）

因为一些原因，长时间运行的操作花费了冗长的时间（需要取消，避免占用资源）；或者不愿意再等待执行结果了

我们会取消异步操作。

为完成目的需要在长时间运行的异步任务中传入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; }

在长时间运行的操作中监测 IsCancellationRequested方法 (当前是否发生取消命令)，这里我倾向去包装一个 TaskCanceledException异常类（给上层方法调用者更多处理的可能性）；当然可以调用 ThrowIfCancellationRequested方法抛出 OperationCanceledException异常。

② 触发取消命令

CancellationToken结构体相当于打入在异步操作内部的楔子，随时等候后方发来的取消命令。

操纵以上CancellationToken状态的对象是 CancellationTokenSource，这个对象是取消操作的命令发布者。

默认的构造函数就支持了超时取消：

// 以下代码 利用 CancellationSource默认构造函数 完成超时取消 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：高阶操作，完成手动取消：

自然我们关注到 CancellationSource 的几个方法，要想在异步操作的时候手动取消操作，需要建立另外的线程等待手动取消操作的指令。

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; } }
// 以上是一个控制台程序，异步接收控制台输入，发出取消命令。

附②：高阶操作，取消 non-Cancellable任务：

有时候，异步操作代码并不提供对 Cancellation的支持，也就是以上长时间运行的异步操作

LongRunningCancellableOperation(int loop, CancellationToken cancellationToken) 并不提供参数2的传入，相当于不允许打入楔子。

这时我们怎样取消这样的non-Cancellable 任务？

可考虑利用 Task.WhenAny( params tasks) 操作曲线取消：

利用TaskCompletionSource 注册异步可取消任务
等待待non-cancellable 操作和以上建立的异步取消操作

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; }

像上面代码一样执行取消命令：

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; } }