取消任务列表

2025-04-04

如果不想等待异步控制台应用程序完成，可以取消它。通过参考本主题中的示例，您可以为用于下载网站列表内容的应用程序添加取消功能。可以通过将 CancellationTokenSource 实例与每个任务相关联来取消许多任务。如果选择 Enter 键，则取消尚未完成的所有任务。

本教程涉及：

创建 .NET 控制台应用程序
编写支持取消的异步应用程序
演示发出取消信号

先决条件

创建示例应用程序

创建新的 .NET Core 控制台应用程序。可以通过 dotnet new console 命令或 Visual Studio 创建一个。在最喜欢的代码编辑器中打开 Program.cs 文件。

使用指令替换

将现有 using 指令替换为以下声明：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

添加字段

在类定义中 Program ，添加以下三个字段：

static readonly CancellationTokenSource s_cts = new CancellationTokenSource();

static readonly HttpClient s_client = new HttpClient
{
    MaxResponseContentBufferSize = 1_000_000
};

static readonly IEnumerable<string> s_urlList = new string[]
{
    "https://learn.microsoft.com",
    "https://learn.microsoft.com/aspnet/core",
    "https://learn.microsoft.com/azure",
    "https://learn.microsoft.com/azure/devops",
    "https://learn.microsoft.com/dotnet",
    "https://learn.microsoft.com/dynamics365",
    "https://learn.microsoft.com/education",
    "https://learn.microsoft.com/enterprise-mobility-security",
    "https://learn.microsoft.com/gaming",
    "https://learn.microsoft.com/graph",
    "https://learn.microsoft.com/microsoft-365",
    "https://learn.microsoft.com/office",
    "https://learn.microsoft.com/powershell",
    "https://learn.microsoft.com/sql",
    "https://learn.microsoft.com/surface",
    "https://learn.microsoft.com/system-center",
    "https://learn.microsoft.com/visualstudio",
    "https://learn.microsoft.com/windows",
    "https://learn.microsoft.com/maui"
};

CancellationTokenSource 用于向 CancellationToken 发出请求取消的信号。公开 HttpClient 发送 HTTP 请求和接收 HTTP 响应的功能。该s_urlList包含应用程序计划处理的所有URL。

更新应用程序入口点

控制台应用程序的主要入口点是 Main 方法。将现有方法替换为以下内容：

static async Task Main()
{
    Console.WriteLine("Application started.");
    Console.WriteLine("Press the ENTER key to cancel...\n");

    Task cancelTask = Task.Run(() =>
    {
        while (Console.ReadKey().Key != ConsoleKey.Enter)
        {
            Console.WriteLine("Press the ENTER key to cancel...");
        }

        Console.WriteLine("\nENTER key pressed: cancelling downloads.\n");
        s_cts.Cancel();
    });

    Task sumPageSizesTask = SumPageSizesAsync();

    Task finishedTask = await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });
    if (finishedTask == cancelTask)
    {
        // wait for the cancellation to take place:
        try
        {
            await sumPageSizesTask;
            Console.WriteLine("Download task completed before cancel request was processed.");
        }
        catch (TaskCanceledException)
        {
            Console.WriteLine("Download task has been cancelled.");
        }
    }

    Console.WriteLine("Application ending.");
}

目前将已更新的 Main 方法视为异步 main 方法，这允许将异步入口点引入可执行文件中。它会将一些说明性消息写入控制台，然后声明一个名为TaskcancelTask实例，该实例将读取控制台键笔划。如果按下 Enter 键，则会调用该 CancellationTokenSource.Cancel() 键。这将发出取消信号。下一步，从 SumPageSizesAsync 方法分配 sumPageSizesTask 变量。接着，这两个任务都会传递给 Task.WhenAny(Task[])，当任一任务完成时便会继续执行。

下一个代码块可确保在处理取消之前应用程序不会退出。如果要完成的第一个任务是 cancelTask，则等待 sumPageSizeTask。如果已取消，则等待时会引发 System.Threading.Tasks.TaskCanceledException。块捕获该异常，并输出消息。

创建异步计算页大小总和的方法

在 Main 方法下方，添加 SumPageSizesAsync 方法：

static async Task SumPageSizesAsync()
{
    var stopwatch = Stopwatch.StartNew();

    int total = 0;
    foreach (string url in s_urlList)
    {
        int contentLength = await ProcessUrlAsync(url, s_client, s_cts.Token);
        total += contentLength;
    }

    stopwatch.Stop();

    Console.WriteLine($"\nTotal bytes returned:  {total:#,#}");
    Console.WriteLine($"Elapsed time:          {stopwatch.Elapsed}\n");
}

该方法从实例化和启动 Stopwatch 开始。然后，它会循环访问和调用ProcessUrlAsync中的每个 s_urlList URL。每次迭代时，将 s_cts.Token 传入 ProcessUrlAsync 方法，代码将返回一个 Task<TResult>整数，其中 TResult 为整数：

int total = 0;
foreach (string url in s_urlList)
{
    int contentLength = await ProcessUrlAsync(url, s_client, s_cts.Token);
    total += contentLength;
}

添加进程方法

在SumPageSizesAsync方法下方添加以下ProcessUrlAsync方法：

static async Task<int> ProcessUrlAsync(string url, HttpClient client, CancellationToken token)
{
    HttpResponseMessage response = await client.GetAsync(url, token);
    byte[] content = await response.Content.ReadAsByteArrayAsync(token);
    Console.WriteLine($"{url,-60} {content.Length,10:#,#}");

    return content.Length;
}

对于任何给定的 URL，该方法都将使用提供的 client 实例以 byte[] 形式来获取响应。 CancellationToken 实例被传递到 HttpClient.GetAsync(String, CancellationToken) 和 HttpContent.ReadAsByteArrayAsync() 方法中。 token 用于注册请求取消。在将 URL 和长度写入控制台后，返回长度。

应用程序输出示例

Application started.
Press the ENTER key to cancel...

https://learn.microsoft.com                                       37,357
https://learn.microsoft.com/aspnet/core                           85,589
https://learn.microsoft.com/azure                                398,939
https://learn.microsoft.com/azure/devops                          73,663
https://learn.microsoft.com/dotnet                                67,452
https://learn.microsoft.com/dynamics365                           48,582
https://learn.microsoft.com/education                             22,924

ENTER key pressed: cancelling downloads.

Application ending.

完整示例

以下代码是示例 Program.cs 文件的完整文本。

using System.Diagnostics;

class Program
{
    static readonly CancellationTokenSource s_cts = new CancellationTokenSource();

    static readonly HttpClient s_client = new HttpClient
    {
        MaxResponseContentBufferSize = 1_000_000
    };

    static readonly IEnumerable<string> s_urlList = new string[]
    {
            "https://learn.microsoft.com",
            "https://learn.microsoft.com/aspnet/core",
            "https://learn.microsoft.com/azure",
            "https://learn.microsoft.com/azure/devops",
            "https://learn.microsoft.com/dotnet",
            "https://learn.microsoft.com/dynamics365",
            "https://learn.microsoft.com/education",
            "https://learn.microsoft.com/enterprise-mobility-security",
            "https://learn.microsoft.com/gaming",
            "https://learn.microsoft.com/graph",
            "https://learn.microsoft.com/microsoft-365",
            "https://learn.microsoft.com/office",
            "https://learn.microsoft.com/powershell",
            "https://learn.microsoft.com/sql",
            "https://learn.microsoft.com/surface",
            "https://learn.microsoft.com/system-center",
            "https://learn.microsoft.com/visualstudio",
            "https://learn.microsoft.com/windows",
            "https://learn.microsoft.com/maui"
    };

    static async Task Main()
    {
        Console.WriteLine("Application started.");
        Console.WriteLine("Press the ENTER key to cancel...\n");

        Task cancelTask = Task.Run(() =>
        {
            while (Console.ReadKey().Key != ConsoleKey.Enter)
            {
                Console.WriteLine("Press the ENTER key to cancel...");
            }

            Console.WriteLine("\nENTER key pressed: cancelling downloads.\n");
            s_cts.Cancel();
        });

        Task sumPageSizesTask = SumPageSizesAsync();

        Task finishedTask = await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });
        if (finishedTask == cancelTask)
        {
            // wait for the cancellation to take place:
            try
            {
                await sumPageSizesTask;
                Console.WriteLine("Download task completed before cancel request was processed.");
            }
            catch (OperationCanceledException)
            {
                Console.WriteLine("Download task has been cancelled.");
            }
        }

        Console.WriteLine("Application ending.");
    }

    static async Task SumPageSizesAsync()
    {
        var stopwatch = Stopwatch.StartNew();

        int total = 0;
        foreach (string url in s_urlList)
        {
            int contentLength = await ProcessUrlAsync(url, s_client, s_cts.Token);
            total += contentLength;
        }

        stopwatch.Stop();

        Console.WriteLine($"\nTotal bytes returned:  {total:#,#}");
        Console.WriteLine($"Elapsed time:          {stopwatch.Elapsed}\n");
    }

    static async Task<int> ProcessUrlAsync(string url, HttpClient client, CancellationToken token)
    {
        HttpResponseMessage response = await client.GetAsync(url, token);
        byte[] content = await response.Content.ReadAsByteArrayAsync(token);
        Console.WriteLine($"{url,-60} {content.Length,10:#,#}");

        return content.Length;
    }
}

另请参阅

后续步骤

在一段时间后取消异步任务（C#）

通过