在一段时间后取消异步任务

  • 项目

如果不希望等待操作结束，可使用 CancellationTokenSource.CancelAfter 方法在一段时间后取消异步操作。 此方法会计划取消未在 CancelAfter 表达式指定的时间段内完成的任何关联任务。

此示例添加到取消任务列表 (C#) 中开发的代码，以下载网站列表并显示每个网站的内容长度。

本教程涉及：

  • 更新现有的 .NET 控制台应用程序
  • 计划取消

必备条件

本教程需要的内容如下：

更新应用程序入口点

将现有的 Main 方法替换为以下内容：

static async Task Main()
{
    Console.WriteLine("Application started.");

    try
    {
        s_cts.CancelAfter(3500);

        await SumPageSizesAsync();
    }
    catch (OperationCanceledException)
    {
        Console.WriteLine("\nTasks cancelled: timed out.\n");
    }
    finally
    {
        s_cts.Dispose();
    }

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

更新后的 Main 方法将一些说明性消息写入控制台。 在 try-catch 内，对 CancellationTokenSource.CancelAfter(Int32) 调用安排取消。 一段时间后将发出取消信号。

接下来，等待 SumPageSizesAsync 方法。 如果处理所有 URL 的速度比计划取消的速度快，应用程序将结束。 但如果在处理所有 URL 之前触发了计划取消，则会引发 OperationCanceledException。

示例应用程序输出

Application started.

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

Tasks cancelled: timed out.

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://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.");

        try
        {
            s_cts.CancelAfter(3500);

            await SumPageSizesAsync();
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("\nTasks cancelled: timed out.\n");
        }
        finally
        {
            s_cts.Dispose();
        }

        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