Task.WaitAll不等待其他异步方法 [英] Task.WaitAll not waiting on other async methods

问题描述

我正在使用Microsoft.Bcl库(不带Task.WhenAll)的可移植类库异步检索一些rss文章.每篇文章都有一个rss注释的URL，我也需要异步检索该注释.

I'm asynchronously retrieving some rss articles with my Portable Class Library that uses the Microsoft.Bcl library (which doesn't have Task.WhenAll). Each article has a url to rss comments that I need to asynchronously retrieve as well.

下面的代码是我的库.我调用GetArticles()，但是它不返回任何创建调用GetComments()以异步获取评论的任务列表的任何内容.

The code below is my library. I call GetArticles() but it does not return any of the which creates a list of tasks that call GetComments() to asynchronously get the comments.

我尝试在GetArticles中使用Task.WaitAll等待注释，但它不会阻塞线程.任何帮助将不胜感激.

I've tried using Task.WaitAll in GetArticles to wait for the comments but it does not block the thread. Any help would be appreciated.

    private const string ArticlesUri = "";

    public async Task<List<ArticleBrief>> GetArticles()
    {
        var results = new List<ArticleBrief>();
        try
        {
            var wfw = XNamespace.Get("http://wellformedweb.org/CommentAPI/");
            var media = XNamespace.Get("http://search.yahoo.com/mrss/");
            var dc = XNamespace.Get("http://purl.org/dc/elements/1.1/");

            var t = await WebHttpRequestAsync(ArticlesUri);
            StringReader stringReader = new StringReader(t);
            using (var xmlReader = System.Xml.XmlReader.Create(stringReader))
            {
                var doc = System.Xml.Linq.XDocument.Load(xmlReader);
                results = (from e in doc.Element("rss").Element("channel").Elements("item")
                           select
                               new ArticleBrief()
                               {
                                   Title = e.Element("title").Value,
                                   Description = e.Element("description").Value,
                                   Published = Convert.ToDateTime(e.Element("pubDate").Value),
                                   Url = e.Element("link").Value,
                                   CommentUri = e.Element(wfw + "commentRss").Value,
                                   ThumbnailUri = e.Element(media + "thumbnail").FirstAttribute.Value,
                                   Categories = GetCategoryElements(e.Elements("category")),
                                   Creator = e.Element(dc + "creator").Value
                               }).ToList();

            }

            var tasks = new Queue<Task>();
            foreach (var result in results)
            {
                tasks.Enqueue(
                    Task.Factory.StartNew(async ()=>
                        {
                            result.Comments = await GetComments(result.CommentUri);
                        }
                    ));
            }

            Task.WaitAll(tasks.ToArray());
        }
        catch (Exception ex)
        {
            // should do some other
            // logging here. for now pass off
            // exception to callback on UI
            throw ex;
        }

        return results;
    }

    public async Task<List<Comment>> GetComments(string uri)
    {
        var results = new List<Comment>();
        try
        {
            var wfw = XNamespace.Get("http://wellformedweb.org/CommentAPI/");
            var media = XNamespace.Get("http://search.yahoo.com/mrss/");
            var dc = XNamespace.Get("http://purl.org/dc/elements/1.1/");

            var t = await WebHttpRequestAsync(uri);
            StringReader stringReader = new StringReader(t);
            using (var xmlReader = System.Xml.XmlReader.Create(stringReader))
            {
                var doc = System.Xml.Linq.XDocument.Load(xmlReader);
                results = (from e in doc.Element("rss").Element("channel").Elements("item")
                           select
                               new Comment()
                               {
                                   Description = e.Element("description").Value,
                                   Published = Convert.ToDateTime(e.Element("pubDate").Value),
                                   Url = e.Element("link").Value,
                                   Creator = e.Element(dc + "creator").Value
                               }).ToList();
            }
        }
        catch (Exception ex)
        {
            // should do some other
            // logging here. for now pass off
            // exception to callback on UI
            throw ex;
        }

        return results;
    }

    private static async Task<string> WebHttpRequestAsync(string url)
    {
        //TODO: look into getting 
        var request = WebRequest.Create(url);
        request.Method = "GET";

        var response = await request.GetResponseAsync();
        return ReadStreamFromResponse(response);
    }

    private static string ReadStreamFromResponse(WebResponse response)
    {
        using (Stream responseStream = response.GetResponseStream())
        using (StreamReader sr = new StreamReader(responseStream))
        {
            string strContent = sr.ReadToEnd();
            return strContent;
        }
    }

    private List<string> GetCategoryElements(IEnumerable<XElement> categories)
    {
        var listOfCategories = new List<string>();

        foreach (var category in categories)
        {
            listOfCategories.Add(category.Value);
        }

        return listOfCategories;
    }

解决方案中的更新代码，只是在Enqueue方法上添加了.UnWrap():

Updated Code from Solution, just added .UnWrap() on the Enqueue method:

            var tasks = new Queue<Task>();
            foreach (var result in results)
            {
                tasks.Enqueue(
                    Task.Factory.StartNew(async ()=>
                        {
                            result.Comments = await GetComments(result.CommentUri);
                        }
                    ).Unwrap());
            }

            Task.WaitAll(tasks.ToArray());

推荐答案

正在适当等待.问题是您正在创建一个Task，该Task创建了另一个任务(即StartNew返回一个Task<Task>，而您只在等待外部的Task，它很快就完成了(它在内部Task完成之前就已经完成了) )).

It is waiting appropriately. The problem is that you are creating a Task which creates another task (i.e. StartNew is returning a Task<Task> and you are only waiting on the outer Task which completes rather quickly (it completes before the inner Task is complete)).

问题将是:

• 您真的想要这个内在的任务吗?
  • 如果是，则可以使用Task.Unwrap获取代表内部Task和外部Task均已完成的代理任务，并使用它来等待.
  • 如果否，那么您可以在StartNew中删除async/await的使用，以便不存在内部任务(我认为这是首选，尚不清楚为什么需要内部任务). /li>
  • Do you really want that inner task?
    • If yes, then you can use Task.Unwrap to get a proxy task that represents the completion of both the inner and outer Task and use that to Wait on.
    • If no, then you could remove the use of async/await in StartNew so that there is not an inner task (I think this would be prefered, it's not clear why you need the inner task).

    顺便说一句，如果您不使用C#5，请注意关闭foreach变量result请参见

    As an aside, if you are not using C# 5, then watch out for closing over the foreach variable result See

    • 已更改foreach对变量的使用在C#5中?和
    • Has foreach's use of variables been changed in C# 5?, and
    • http://blogs.msdn.com/b/ericlippert/archive/2009/11/12/closing-over-the-loop-variable-considered-harmful.aspx)

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