使用 CancellationToken 和 Ctrl+C 取消多个 HttpClient 调用

Question

我试图在命令行应用程序中取消通过共享HttpClient使用完整框架、.net 4.7.1、C# 7.3、VS 2017 发出的多个异步 Web 请求 (GET)。CancellationTokens

我的示例运行几个并行任务，每个任务都使用异步不断通过 Http 下载一些数据，GetAsync()但ReadAsStringAsync()使用 example 得到相同的结果ReadAsByteArrayAsync()。

终止是通过挂接Console.CancelKeyPress并取消我的CancellationTokenSource.

尽管由于某种原因这看起来很简单，但我无法理解它并想出一个产生可靠结果的解决方案。有时，一切都会按预期关闭，即所有任务都完成（取消），但更常见的是，不关闭只是看似挂起。在不使用调试器（带/不带 DEBUG）的情况下运行会终止应用程序，但不会以我预期的方式终止。任务越少意味着干净关闭的可能性越大。

当处于“挂起”状态时暂停调试中的所有线程似乎表明某些线程被卡住GetAsync()，但很难准确地看到发生了什么。

实际上，应用程序如何退出并不重要，但我想了解这一点，并能够一致地产生干净且受控的关闭，对我来说，使用这种构造似乎是可能的，但我很可能错过了一些细节。

using System;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

namespace HttpClientAsyncTest
{
    class Program
    {
        static async Task<int> Main()
        {
            using (var cancellationTokenSource = new CancellationTokenSource())
            {
                Console.CancelKeyPress += (sender, a) =>
                {
                    Console.WriteLine("Stopped by user");
                    cancellationTokenSource.Cancel();
                };

                var task = RunAsync(cancellationTokenSource);
                Console.WriteLine("Running - Press CTRL+C to stop");
                await task;
            }

            Console.WriteLine("All done, exiting");

            return 0;
        }

        private static async Task RunAsync(CancellationTokenSource cts)
        {
            using (var client = new HttpClient())
            {
                try
                {
                    var tasks = Enumerable.Range(1, 10).Select(id => ProcessBatchAsync(client, id, cts.Token));

                    await Task.WhenAll(tasks);
                }
                catch (OperationCanceledException)
                {
                    Console.WriteLine("Operation cancelled somehow");
                }
            }
        }

        private static async Task ProcessBatchAsync(HttpClient client, int id, CancellationToken cancellationToken)
        {
            while (true)
                await ProcessNextBatchAsync(client, id, cancellationToken);
        }

        private static async Task ProcessNextBatchAsync(HttpClient client, int id, CancellationToken cancellationToken)
        {
            using (var response = await client.GetAsync("http://some.payload.to.request", cancellationToken))
            {
                if (response.StatusCode == HttpStatusCode.NotFound)
                    return;

                var data = await response.Content.ReadAsStringAsync();
                Console.WriteLine($"Id: {id} downloaded {data.Length} chars");
            }
        }
    }
}

我希望在按 Ctrl-C 时看到类似的内容：

Running - Press CTRL+C to stop
Id: 6 downloaded 475357 chars
Id: 2 downloaded 475141 chars
Id: 3 downloaded 474927 chars
Id: 5 downloaded 474457 chars
Id: 8 downloaded 474524 chars
Id: 4 downloaded 474643 chars
Id: 7 downloaded 475133 chars
Id: 9 downloaded 475316 chars
Stopped by user
Operation cancelled somehow
All done, exiting
Press any key to continue . . .

但大多数情况下我得到的都是这样的：

Running - Press CTRL+C to stop
Id: 3 downloaded 474927 chars
Id: 8 downloaded 474524 chars
Id: 5 downloaded 474457 chars
Id: 9 downloaded 475316 chars
Id: 6 downloaded 475357 chars
Id: 7 downloaded 474952 chars
Id: 2 downloaded 475513 chars
Stopped by user
Id: 4 downloaded 475457 chars
Id: 7 downloaded 475133 chars
^CPress any key to continue . . .

Answer 1

好吧，我通过 StackOverflow 的橡皮鸭再次成功了。

Ctrl+C如果没有通过 取消，则终止应用程序，因此它与或 的args.Cancel = true关系绝对为零。HttpClientCancellationToken

修复方法很简单：

Console.CancelKeyPress += (sender, args) =>
{
    args.Cancel = true;
    Console.WriteLine("Stopped by user");
    cancellationTokenSource.Cancel(true);
};