TcpClient.ConnectAsync 方法的取消支持

Question

我希望获得一些意见，以增加我正在处理的部分的弹性，该部分涉及异步创建多个 TcpClient 连接到各种服务器，一旦成功建立 TCP 连接，然后将 TcpClient 存储在排序中集合（只是要求的摘录）

这是我目前所拥有的：

internal class Program
{
    private static readonly TcpClient _client = new TcpClient(AddressFamily.InterNetwork);
    private static SslStream _stream;

    private static async Task Main(string[] args)
    {
        var timeoutTask = Task.Delay(5000);
        var connectTask = _client.ConnectAsync("10.14.16.24", 56564);

        var completedTask = await Task.WhenAny(timeoutTask, connectTask);
        if (completedTask == timeoutTask)
            _client.Dispose();

        try
        {
            await connectTask;
            _stream = new SslStream(_client.GetStream(), false);
        }
        catch (SocketException e)
        {
            Console.WriteLine(e.Message);
        }
    }
}

问题： 如果发生超时，connectTask 状态仍为“WaitingForActivation”，直到ConnectAsync 方法返回Faulted 或RanToCompletion。怎么取消这个任务，知道ConnectAsync不支持CancellationToken

欢迎提出任何建议或改进。 谢谢

Answer 1

您的里程可能会根据您使用的.net 而有所不同

using System;
using System.Net.Sockets;
using System.Threading;
using System.Threading.Tasks;

namespace TcpClientCancellation
{
    public static class Extensions
    {
        public static async Task ConnectAsync(this TcpClient tcpClient, string host, int port, CancellationToken cancellationToken) {
            if (tcpClient == null) {
                throw new ArgumentNullException(nameof(tcpClient));
            }

            cancellationToken.ThrowIfCancellationRequested();

            using (cancellationToken.Register(() => tcpClient.Close())) {
                try {
                    cancellationToken.ThrowIfCancellationRequested();

                    await tcpClient.ConnectAsync(host, port).ConfigureAwait(false);
                } catch (NullReferenceException) when (cancellationToken.IsCancellationRequested) {
                    cancellationToken.ThrowIfCancellationRequested();
                }
            }
        }
    }
}

Answer 2

作为@Kelly Elton 答案的扩展，您似乎应该抓住

• 在 .NET Framework 4.x 中NullReferenceException
• 在 .NET Core 2.x 和 3.x 中ObjectDisposedException
• 在.NET 5中SocketException

所以也许正确的 catch 语句是

#if NET5_0 //_OR_GREATER?
catch (SocketException ex) when (ct.IsCancellationRequested)
#elif NETCOREAPP2_0_OR_GREATER
catch (ObjectDisposedException ex) when (ct.IsCancellationRequested)
#elif NETFRAMEWORK && NET40_OR_GREATER
catch (NullReferenceException ex) when (ct.IsCancellationRequested)
#else
#error Untested target framework, use with care!
catch (???) when (ct.IsCancellationRequested)
#endif

Answer 3

我正在使用这些通用方法；他们可以为任何异步任务添加超时和取消令牌。您需要使用 using var tcpClient 来确保在取消异常引发后处理客户端或主动处理您的客户端

如果您发现任何问题，请告诉我，以便我进行相应的修复。

public static async Task<T> RunTask<T>(Task<T> task, int timeout = 0, CancellationToken cancellationToken = default)
{
    await RunTask((Task)task, timeout, cancellationToken);
    return await task;
}

public static async Task RunTask(Task task, int timeout = 0, CancellationToken cancellationToken = default)
{
    if (timeout == 0) timeout = -1;

    var timeoutTask = Task.Delay(timeout, cancellationToken);
    await Task.WhenAny(task, timeoutTask);

    cancellationToken.ThrowIfCancellationRequested();
    if (timeoutTask.IsCompleted)
        throw new TimeoutException();

    await task;
}

用法

using var tcpClient = new TcpClient();
await RunTask(tcpClient.ConnectAsync("yourhost.com", 443), cancellationToken: CancellationToken.None);