多个网络请求，在等待最后一个响应时启动另一个请求。 C＃

Question

我正在尝试从需要很长时间才能加载完整响应（非常长的日志文件）的网站发出多个 Web 请求。我想实现一种方法，该方法将在等待其他请求响应的同时继续创建 Web 请求并加快处理速度。

尝试实现异步等待方法，但我不认为是在创建同时调用，而是运行同步调用。我对如何实现异步等待感到很困惑，我看了其他类似的问题但仍然不是很清楚，我怎么能把这段代码改成运行 async?
这是我的代码：

        public static async Task<String> MakeRequestAsync(String url)
    {
        String responseText = null;

            try
            {
                HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
                request.Proxy = null;
                WebResponse response = request.GetResponse();
                Stream responseStream = response.GetResponseStream();
                string sr = new StreamReader(responseStream).ReadToEnd();
                response.Close();
                responseStream.Close();
                responseText = sr;
            }

            catch (Exception e)
            {
                Console.WriteLine("Error: " + e.Message);
            }
        return responseText;
    }

..

private void button_test_Click(object sender, EventArgs e)
    {
         for (int n = 0; n < 5; n++)  //multiple requests
            {
                String a = AsyncTest().Result;
            }

    }

    private async Task<String> AsyncTest()
    {
        String b = await MakeRequestAsync(url);
        return b;
    }

还有任何关于如何加快此过程的建议都很棒。

Answer 1

None 个 IO 调用是 async。 WebRequest 也没有 async/await 模式，您可以使用 BeginGetResponse 和 AsyncCallback 获得 async 行为，但还有更好的方法。而是使用 HttpClient，这是进行 http 调用的推荐方式。

即

public static async Task<String> MakeRequestAsync(string url)
{

    using(var client = new HttpClient()) // don't do this, inject it as a singleton
    {
       using (var response = await client.GetAsync(url)
          return await response.Content.ReadAsStringAsync();
    }  
}

然后将您的 Click 处理程序更改为 async void:

private async void button_test_Click(object sender, EventArgs e)
{
     var tasks = new List<Task<string>>();
     for (int n = 0; n < 5; n++)  //multiple requests
     {
        tasks.Add(MakeRequestAsync(url));
     }
     await Task.WhenAll(tasks); // wait for all of them to complete;
     foreach(var task in tasks)
     {
         var str = await task; // or task.Result, won't block, already completed;
     }
}

Answer 2

您应该使用 Microsoft 的 Reactive Framework（又名 Rx）- NuGet System.Reactive 并添加 using System.Reactive.Linq; - 然后您可以这样做：

var query =
    from n in Observable.Range(0, 5)
    from r in Observable.Using(
        () => new WebClient(),
        wc => Observable.Start(() => wc.DownloadString($"{url}{n}")))
    select new { n, r };

IDisposable subscription =
    query
        .ToArray()
        .Select(xs => xs.OrderBy(x => x.n).Select(x => x.r).ToArray())
        .Subscribe(rs =>
        {
            /* Do something with the results */
        });

都是多线程非阻塞的

如果您不关心返回结果的顺序，那么只需这样做：

var query =
    from n in Observable.Range(0, 5)
    from r in Observable.Using(
        () => new WebClient(),
        wc => Observable.Start(() => wc.DownloadString($"{url}{n}")))
    select r;

IDisposable subscription =
    query
        .Subscribe(r =>
        {
            /* Do something with each result */
        });

如果您需要取消查询，调用 subscription.Dispose() 将停止查询。

您可以这样编写 MakeRequestAsync 方法：

public static async Task<String> MakeRequestAsync(String url)
{
    return await
        Observable.Using(
            () => new WebClient(),
            wc => Observable.Start(() => wc.DownloadString(url)));
}

多个网络请求，在等待最后一个响应时启动另一个请求。 C＃

Multiple web requests, start another request while waiting for response on last one. C#

c#

multithreading

httpwebrequest

async-await