在一段時間後取消非同步工作

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如果不想等候作業完成，則可以使用 CancellationTokenSource.CancelAfter 方法，在一段時間之後取消非同步作業。 此方法會排程取消未在 CancelAfter 運算式所指定之期間內完成的任何相關工作。

此範例會新增至取消工作清單 (C#) 中所開發的程式碼來下載網站清單，以及顯示每個網站內容的長度。

此教學課程涵蓋：

  • 更新現有的 .NET 主控台應用程式
  • 排程取消

必要條件

本教學課程需要下列：

更新應用程式進入點

將現有的 Main 方法取代為以下內容：

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.");
}

更新的 Main 方法會將一些指示訊息寫入主控台。 在 try-catch 中，CancellationTokenSource.CancelAfter(Int32) 的呼叫會排程取消。 這會在一段時間後發出取消訊號。

接著，SumPageSizesAsync 方法會進入等待。 如果處理所有 URL 的速度比排程取消更快，應用程式就會結束。 不過，如果在處理所有 URL 之前觸發排程取消，則會擲回 OperationCanceledException。

範例應用程式輸出

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.

完整範例

下列程式碼是範例 Program.cs 檔案的完整文字。

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/xamarin"
    };

    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;
    }
}

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