如何等待多个异步HTTP请求 [英] How to wait for multiple async http request

问题描述

我想问一下如何等待多个异步http请求.

I'd like to ask about how to wait for multiple async http requests.

我的代码是这样的:

    public void Convert(XDocument input, out XDocument output)
    {
        var ns = input.Root.Name.Namespace;

        foreach (var element in input.Root.Descendants(ns + "a"))
        {
            Uri uri = new Uri((string)element.Attribute("href"));

            var wc = new WebClient();
            wc.OpenReadCompleted += ((sender, e) =>
            {
                element.Attribute("href").Value = e.Result.ToString();
            }
            );
            wc.OpenReadAsync(uri);
        }

        //I'd like to wait here until above async requests are all completed.
        output = input;  
    }

有人知道这个的解决方案吗?

Dose anyone know a solution for this?

推荐答案

有一个文章由Scott撰写Hanselman 在其中描述了如何执行非阻塞请求.滚动到末尾，有一个 public Task< bool>.ValidateUrlAsync(string url)方法.

There is an article by Scott Hanselman in which he describes how to do non blocking requests. Scrolling to the end of it, there is a public Task<bool> ValidateUrlAsync(string url) method.

您可以像这样修改它(在响应阅读方面可能会更强大)

You could modify it like this (could be more robust about response reading)

public Task<string> GetAsync(string url)
{
    var tcs = new TaskCompletionSource<string>();
    var request = (HttpWebRequest)WebRequest.Create(url);
    try
    {
        request.BeginGetResponse(iar =>
        {
            HttpWebResponse response = null;
            try
            {
                response = (HttpWebResponse)request.EndGetResponse(iar);
                using(var reader = new StreamReader(response.GetResponseStream()))
                {
                    tcs.SetResult(reader.ReadToEnd());
                }                    
            }
            catch(Exception exc) { tcs.SetException(exc); }
            finally { if (response != null) response.Close(); }
        }, null);
    }
    catch(Exception exc) { tcs.SetException(exc); }
    return tsc.Task; 
}

因此，您可以像这样使用它

So with this in hand, you could then use it like this

var urls=new[]{"url1","url2"};
var tasks = urls.Select(GetAsync).ToArray();
var completed = Task.Factory.ContinueWhenAll(tasks,
                    completedTasks =>{
                                          foreach(var result in completedTasks.Select(t=>t.Result))
                                          {
                                              Console.WriteLine(result);
                                          }
                                      });
completed.Wait();
//anything that follows gets executed after all urls have finished downloading

希望这会使您朝着正确的方向前进.

Hope this puts you in the right direction.

PS.这可能很清楚，而无需使用async/await

PS. this is probably as clear as it can get without using async/await

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