c# - 执行异步 Web 请求时出现问题

Question

谁能告诉我这段简单的代码做错了什么？当我运行它时，它会挂起

using (Stream postStream = request.EndGetRequestStream(asynchronousResult))

如果我注释掉 requestState.Wait.WaitOne();行代码正确执行但显然不等待响应。我猜对 EndGetRequestStream 的调用以某种方式使我返回到主线程的上下文？我很确定我的代码与示例基本相同 ( MSDN Documentation )

using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.IO;
using System.Text;

namespace SBRemoteClient
{
    public class JSONClient
    {

        public string ExecuteJSONQuery(string url, string query)
        {
            System.Uri uri = new Uri(url);
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
            request.Method = "POST";
            request.Accept = "application/json";
            byte[] requestBytes = Encoding.UTF8.GetBytes(query);
            RequestState requestState = new RequestState(request, requestBytes);
            IAsyncResult resultRequest = request.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), requestState);
            requestState.Wait.WaitOne();
            IAsyncResult resultResponse = (IAsyncResult)request.BeginGetResponse(new AsyncCallback(GetResponseStreamCallback), requestState);
            requestState.Wait.WaitOne();

            return requestState.Response;
        }

        private static void GetRequestStreamCallback(IAsyncResult asynchronousResult)
        {
            try
            {
                RequestState requestState = (RequestState)asynchronousResult.AsyncState;
                HttpWebRequest request = requestState.Request;
                using (Stream postStream = request.EndGetRequestStream(asynchronousResult))
                {
                    postStream.Write(requestState.RequestBytes, 0, requestState.RequestBytes.Length);
                }
                requestState.Wait.Set();
            }
            catch (Exception e) {
                Console.Out.WriteLine(e);
            }
        }

        private static void GetResponseStreamCallback(IAsyncResult asynchronousResult)
        {
            RequestState requestState = (RequestState)asynchronousResult.AsyncState;
            HttpWebRequest request = requestState.Request;
            using (HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult))
            {
                using (Stream responseStream = response.GetResponseStream())
                {
                    using (StreamReader streamRead = new StreamReader(responseStream))
                    {
                        requestState.Response = streamRead.ReadToEnd();
                        requestState.Wait.Set();
                    }
                }
            }
        }
    }
}

Answer 1

几件事:

1. 我认为您不能重复使用同一个请求对象。也就是说，您可以为每个 HttpWebRequest 实例调用一次 BeginGetRequestStream。如果你想执行两个请求(调用服务器)，你需要两个 HttpWebRequest 实例，每个请求一个。

2. 如果您想要同步行为，您有两个选择:使用 GetRespose 方法或以同步方式使用开始/结束。为此，您不需要将回调传递给 BeginGetRequestStream 方法(您可以传递一个空值)。从 BeginGetRequestStream (IAsyncResult) 获取返回值并将其传递给 EndGetRequestStream 方法:

   AsyncResult resultRequest = request.BeginGetRequestStream(null, null);
   Stream postStream = request.EndGetRequestStream(asynchronousResult)
   

EndGetRequestStream 将阻塞直到请求完成(如果您从 UI 执行此操作很糟糕，但它仍然有效)。