如何：侦听多个取消请求

项目
04/07/2023

此示例展示了如何同时侦听两个取消令牌，以便在其中任意一个令牌发出请求时取消操作。

注意

某些情况下，当启用“仅我的代码”后，Visual Studio 会在引发异常的行中断运行并显示一条错误消息，该消息显示“用户代码未处理异常”。该错误是良性错误。可以按 F5 继续运行，并查看下面示例中所示的异常处理行为。为了阻止 Visual Studio 在第一个错误出现时中断，只需依次转到“工具”、“选项”、“调试”、“常规”下，取消选中“仅我的代码”复选框即可。

示例

在下面的示例中，CreateLinkedTokenSource 方法用于将两个令牌联接为一个令牌。这样一来，可以将此令牌传递到只需要使用一个取消令牌作为参数的方法中。此示例展示了以下常见方案：方法必须同时观察从类外部传入的令牌和类内部生成的令牌。

using System;
using System.Threading;
using System.Threading.Tasks;

class LinkedTokenSourceDemo
{
    static void Main()
    {
        WorkerWithTimer worker = new WorkerWithTimer();
        CancellationTokenSource cts = new CancellationTokenSource();

        // Task for UI thread, so we can call Task.Wait wait on the main thread.
        Task.Run(() =>
        {
            Console.WriteLine("Press 'c' to cancel within 3 seconds after work begins.");
            Console.WriteLine("Or let the task time out by doing nothing.");
            if (Console.ReadKey(true).KeyChar == 'c')
                cts.Cancel();
        });

        // Let the user read the UI message.
        Thread.Sleep(1000);

        // Start the worker task.
        Task task = Task.Run(() => worker.DoWork(cts.Token), cts.Token);

        try
        {
            task.Wait(cts.Token);
        }
        catch (OperationCanceledException e)
        {
            if (e.CancellationToken == cts.Token)
                Console.WriteLine("Canceled from UI thread throwing OCE.");
        }
        catch (AggregateException ae)
        {
            Console.WriteLine("AggregateException caught: " + ae.InnerException);
            foreach (var inner in ae.InnerExceptions)
            {
                Console.WriteLine(inner.Message + inner.Source);
            }
        }

        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
        cts.Dispose();
    }
}

class WorkerWithTimer
{
    CancellationTokenSource internalTokenSource = new CancellationTokenSource();
    CancellationToken internalToken;
    CancellationToken externalToken;
    Timer timer;

    public WorkerWithTimer()
    {
        // A toy cancellation trigger that times out after 3 seconds
        // if the user does not press 'c'.
        timer = new Timer(new TimerCallback(CancelAfterTimeout), null, 3000, 3000);
    }

    public void DoWork(CancellationToken externalToken)
    {
        // Create a new token that combines the internal and external tokens.
        this.internalToken = internalTokenSource.Token;
        this.externalToken = externalToken;

        using (CancellationTokenSource linkedCts =
                CancellationTokenSource.CreateLinkedTokenSource(internalToken, externalToken))
        {
            try
            {
                DoWorkInternal(linkedCts.Token);
            }
            catch (OperationCanceledException)
            {
                if (internalToken.IsCancellationRequested)
                {
                    Console.WriteLine("Operation timed out.");
                }
                else if (externalToken.IsCancellationRequested)
                {
                    Console.WriteLine("Cancelling per user request.");
                    externalToken.ThrowIfCancellationRequested();
                }
            }
        }
    }

    private void DoWorkInternal(CancellationToken token)
    {
        for (int i = 0; i < 1000; i++)
        {
            if (token.IsCancellationRequested)
            {
                // We need to dispose the timer if cancellation
                // was requested by the external token.
                timer.Dispose();

                // Throw the exception.
                token.ThrowIfCancellationRequested();
            }

            // Simulating work.
            Thread.SpinWait(7500000);
            Console.Write("working... ");
        }
    }

    public void CancelAfterTimeout(object? state)
    {
        Console.WriteLine("\r\nTimer fired.");
        internalTokenSource.Cancel();
        timer.Dispose();
    }
}

Imports System.Threading
Imports System.Threading.Tasks

Class LinkedTokenSourceDemo
    Shared Sub Main()
        Dim worker As New WorkerWithTimer()
        Dim cts As New CancellationTokenSource()

        ' Task for UI thread, so we can call Task.Wait wait on the main thread.
        Task.Run(Sub()
                     Console.WriteLine("Press 'c' to cancel within 3 seconds after work begins.")
                     Console.WriteLine("Or let the task time out by doing nothing.")
                     If Console.ReadKey(True).KeyChar = "c"c Then
                         cts.Cancel()
                     End If
                 End Sub)
        ' Let the user read the UI message.
        Thread.Sleep(1000)

        ' Start the worker task.
        Dim t As Task = Task.Run(Sub() worker.DoWork(cts.Token), cts.Token)
        Try
            t.Wait()
        Catch ae As AggregateException
            For Each inner In ae.InnerExceptions
                Console.WriteLine(inner.Message)
            Next
        End Try

        Console.WriteLine("Press any key to exit.")
        Console.ReadKey()
        cts.Dispose()
    End Sub
End Class

Class WorkerWithTimer
    Dim internalTokenSource As CancellationTokenSource
    Dim token As CancellationToken
    Dim timer As Timer

    Public Sub New()
        internalTokenSource = New CancellationTokenSource()
        token = internalTokenSource.Token

        ' A toy cancellation trigger that times out after 3 seconds
        ' if the user does not press 'c'.
        timer = New Timer(New TimerCallback(AddressOf CancelAfterTimeout), Nothing, 3000, 3000)
    End Sub

    Public Sub DoWork(ByVal externalToken As CancellationToken)
        ' Create a new token that combines the internal and external tokens.
        Dim internalToken As CancellationToken = internalTokenSource.Token
        Dim linkedCts As CancellationTokenSource =
        CancellationTokenSource.CreateLinkedTokenSource(internalToken, externalToken)
        Using (linkedCts)
            Try
                DoWorkInternal(linkedCts.Token)
            Catch e As OperationCanceledException
                If e.CancellationToken = internalToken Then
                    Console.WriteLine("Operation timed out.")
                ElseIf e.CancellationToken = externalToken Then
                    Console.WriteLine("Canceled by external token.")
                    externalToken.ThrowIfCancellationRequested()
                End If
            End Try
        End Using
    End Sub

    Private Sub DoWorkInternal(ByVal token As CancellationToken)
        For i As Integer = 0 To 1000
            If token.IsCancellationRequested Then
                ' We need to dispose the timer if cancellation
                ' was requested by the external token.
                timer.Dispose()

                ' Output for demonstration purposes.
                Console.WriteLine(vbCrLf + "Cancelling per request.")

                ' Throw the exception.
                token.ThrowIfCancellationRequested()
            End If

            ' Simulating work.
            Thread.SpinWait(7500000)
            Console.Write("working... ")
        Next
    End Sub

    Public Sub CancelAfterTimeout(ByVal state As Object)
        Console.WriteLine(vbCrLf + "Timer fired.")
        internalTokenSource.Cancel()
        timer.Dispose()
    End Sub
End Class

如果链接令牌抛出 OperationCanceledException，传递到异常的令牌是链接令牌，而不是任意前置任务令牌。若要确定取消的是哪个令牌，请直接查看前置任务令牌的状态。

虽然 AggregateException 不得引发，但此示例捕获到它的原因是，在实际情况下，任务委托引发的除 OperationCanceledException 外其他任何异常都包装在 AggregateException 中。

请参阅

托管线程中的取消

如何：侦听多个取消请求

示例

请参阅

反馈

其他资源