在异步请求中使用超时回调

Question

我之前问过这个问题，但我将用提出的解决方案来完成这个问题并提出另一个问题。

我正在使用这个类来进行异步WebRequest：

class HttpSocket
{
    public static void MakeRequest(Uri uri, Action<RequestCallbackState> responseCallback)
    {
        WebRequest request = WebRequest.Create(uri);
        request.Proxy = null;

        Task<WebResponse> asyncTask = Task.Factory.FromAsync<WebResponse>(request.BeginGetResponse, request.EndGetResponse, null);
        ThreadPool.RegisterWaitForSingleObject((asyncTask as IAsyncResult).AsyncWaitHandle, new WaitOrTimerCallback(TimeoutCallback), request, 1000, true);
        asyncTask.ContinueWith(task =>
            {
                WebResponse response = task.Result;
                Stream responseStream = response.GetResponseStream();
                responseCallback(new RequestCallbackState(response.GetResponseStream()));
                responseStream.Close();
                response.Close();
            });
    }

    private static void TimeoutCallback(object state, bool timedOut)
    {
        Console.WriteLine("Timeout: " + timedOut);
        if (timedOut)
        {
            Console.WriteLine("Timeout");
            WebRequest request = (WebRequest)state;
            if (state != null)
            {
                request.Abort();
            }
        }
    }
}

我正在用这段代码测试这个类：

class Program
{
    static void Main(string[] args)
    {
        // Making a request to a nonexistent domain.
        HttpSocket.MakeRequest(new Uri("http://www.google.comhklhlñ"), callbackState =>
            {
                if (callbackState.Exception != null)
                    throw callbackState.Exception;
                Console.WriteLine(GetResponseText(callbackState.ResponseStream));
            });
        Thread.Sleep(100000);
    }

    public static string GetResponseText(Stream responseStream)
    {
        using (var reader = new StreamReader(responseStream))
        {
            return reader.ReadToEnd();
        }
    }
}

一旦执行，立即到达回调，显示“超时：假”并且没有更多的抛出，因此超时不起作用。

这是original thread 中提出的解决方案，但正如您所见，该代码适用于他。

我做错了什么？

编辑：代码使用的其他类：

class RequestCallbackState
{
    public Stream ResponseStream { get; private set; }
    public Exception Exception { get; private set; }

    public RequestCallbackState(Stream responseStream)
    {
        ResponseStream = responseStream;
    }

    public RequestCallbackState(Exception exception)
    {
        Exception = exception;
    }
}

class RequestState
{
    public byte[] RequestBytes { get; set; }
    public WebRequest Request { get; set; }
    public Action<RequestCallbackState> ResponseCallback { get; set; }
}

Answer 1

这种方法有效。我建议切换它以稍微不同地显式处理异常（包括您的超时，以及错误的域名等）。在这种情况下，我将其拆分为单独的延续。

此外，为了使这一点非常明确，我缩短了超时时间，放入了一个“真实”但速度较慢的域，并添加了一个您可以看到的显式超时状态：

using System;
using System.IO;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

class HttpSocket
{
    private const int TimeoutLength = 100;

    public static void MakeRequest(Uri uri, Action<RequestCallbackState> responseCallback)
    {
        WebRequest request = WebRequest.Create(uri);
        request.Proxy = null;

        Task<WebResponse> asyncTask = Task.Factory.FromAsync<WebResponse>(request.BeginGetResponse, request.EndGetResponse, null);
        ThreadPool.RegisterWaitForSingleObject((asyncTask as IAsyncResult).AsyncWaitHandle, new WaitOrTimerCallback(TimeoutCallback), request, TimeoutLength, true);
        asyncTask.ContinueWith(task =>
            {
                WebResponse response = task.Result;
                Stream responseStream = response.GetResponseStream();
                responseCallback(new RequestCallbackState(response.GetResponseStream()));
                responseStream.Close();
                response.Close();
            }, TaskContinuationOptions.NotOnFaulted);
        // Handle errors
        asyncTask.ContinueWith(task =>
            {
                var exception = task.Exception;
                var webException = exception.InnerException;

                // Track whether you cancelled or not... up to you...
                responseCallback(new RequestCallbackState(exception.InnerException, webException.Message.Contains("The request was canceled.")));
            }, TaskContinuationOptions.OnlyOnFaulted);
    }

    private static void TimeoutCallback(object state, bool timedOut)
    {
        Console.WriteLine("Timeout: " + timedOut);
        if (timedOut)
        {
            Console.WriteLine("Timeout");
            WebRequest request = (WebRequest)state;
            if (state != null)
            {
                request.Abort();
            }
        }
    }
}

class RequestCallbackState
{
    public Stream ResponseStream { get; private set; }
    public Exception Exception { get; private set; }

    public bool RequestTimedOut { get; private set; }

    public RequestCallbackState(Stream responseStream)
    {
        ResponseStream = responseStream;
    }

    public RequestCallbackState(Exception exception, bool timedOut = false)
    {
        Exception = exception;
        RequestTimedOut = timedOut;
    }
}

class RequestState
{
    public byte[] RequestBytes { get; set; }
    public WebRequest Request { get; set; }
    public Action<RequestCallbackState> ResponseCallback { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        // Making a request to a nonexistent domain.
        HttpSocket.MakeRequest(new Uri("http://www.tanzaniatouristboard.com/"), callbackState =>
            {
                if (callbackState.RequestTimedOut)
                {
                    Console.WriteLine("Timed out!");
                }
                else if (callbackState.Exception != null)
                    throw callbackState.Exception;
                else
                    Console.WriteLine(GetResponseText(callbackState.ResponseStream));
            });
        Thread.Sleep(100000);
    }

    public static string GetResponseText(Stream responseStream)
    {
        using (var reader = new StreamReader(responseStream))
        {
            return reader.ReadToEnd();
        }
    }
}

这将运行，并适当地显示超时。

Answer 2

使用 2 个不同的类：

class RequestCallbackException : Exception
{
    public RequestCallbackException(Stream responseStream, Exception exception) : base(exception)
    {
    }
}

和

class RequestCallbackStream
{
    public Stream ResponseStream { get; private set; }

    public RequestCallbackState(Stream responseStream)
    {
        ResponseStream = responseStream;
    }
}

您会注意到有时 GetResponseStream() 返回 null，这会立即引发异常

asyncTask.ContinueWith() -->

GetResponseText(callbackState.ResponseStream)-->

using (var reader = new StreamReader(responseStream)) // responseStream is null
{
}