取消任务列表 (C#)

项目
09/22/2022

如果不想等待异步控制台应用程序完成，可以取消该应用程序。通过遵循本主题中的示例，可将取消添加到下载网站内容的应用程序。可通过将 CancellationTokenSource 实例与每个任务进行关联来取消多个任务。如果选择 Enter 键，则将取消所有尚未完成的任务。

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

必备条件

本教程需要的内容如下：

.NET 5 或更高版本的 SDK
集成开发环境 (IDE)
- 建议使用 Visual Studio、Visual Studio Code 或 Visual Studio for Mac

创建示例应用程序

创建新的 .NET Core 控制台应用程序。可通过使用 dotnet new console 命令或从 Visual Studio 进行创建。在你最喜欢的编辑器中打开 Program.cs 文件。

替换 using 语句

将现有 using 语句替换为以下声明：

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 Program 类定义中，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/xamarin"
};

CancellationTokenSource 用于向 CancellationToken 发出请求取消的信号。 HttpClient 公开发送 HTTP 请求和接收 HTTP 响应的能力。 s_urlList 包括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();

    await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });

    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。将几条说明性消息写入控制台，然后声明名为 cancelTask 的 Task 实例，这将读取控制台密钥笔画。如果按 Enter，则会调用 CancellationTokenSource.Cancel()。这将发出取消信号。下一步，从 SumPageSizesAsync 方法分配 sumPageSizesTask 变量。然后，将这两个任务传递到 Task.WhenAny(Task[])，这会在完成两个任务中的任意一个时继续。

创建异步总和页面大小方法

在 Main 方法下，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。然后会在 s_urlList 的每个 URL 中进行循环，并调用 ProcessUrlAsync。对于每次迭代，s_cts.Token 都会传递到 ProcessUrlAsync 方法中，并且代码将返回 Task<TResult>，其中 TResult 是一个整数：

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 实例会传递到 HttpClient.GetAsync(String, CancellationToken) 和 HttpContent.ReadAsByteArrayAsync() 方法中。 token 用于注册请求取消。将 URL 和长度写入控制台后会返回该长度。

示例应用程序输出

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

下列代码是示例的 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://docs.microsoft.com",
            "https://docs.microsoft.com/aspnet/core",
            "https://docs.microsoft.com/azure",
            "https://docs.microsoft.com/azure/devops",
            "https://docs.microsoft.com/dotnet",
            "https://docs.microsoft.com/dynamics365",
            "https://docs.microsoft.com/education",
            "https://docs.microsoft.com/enterprise-mobility-security",
            "https://docs.microsoft.com/gaming",
            "https://docs.microsoft.com/graph",
            "https://docs.microsoft.com/microsoft-365",
            "https://docs.microsoft.com/office",
            "https://docs.microsoft.com/powershell",
            "https://docs.microsoft.com/sql",
            "https://docs.microsoft.com/surface",
            "https://docs.microsoft.com/system-center",
            "https://docs.microsoft.com/visualstudio",
            "https://docs.microsoft.com/windows",
            "https://docs.microsoft.com/xamarin"
    };

    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();

        await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });

        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#)