我正在使用Microsoft.Bcl库（该库没有Task.WhenAll）的便携式类库异步检索一些rss文章。每篇文章都有一个指向rss评论的网址，我也需要异步地获取它们。
下面的代码是我的库。我调用GetArticles()，但它不返回任何内容，它创建了一个任务列表，调用GetComments()以异步获取评论。
我尝试在GetArticles中使用Task.WaitAll等待评论，但它不会阻塞线程。如果您能提供帮助，将不胜感激。
下面的代码是我的库。我调用GetArticles()，但它不返回任何内容，它创建了一个任务列表，调用GetComments()以异步获取评论。
我尝试在GetArticles中使用Task.WaitAll等待评论，但它不会阻塞线程。如果您能提供帮助，将不胜感激。
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;
}
更新解决方案的代码，只需在Enqueue方法上添加.UnWrap（）：
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());
Task.Factory.StartNew(async ()=>，那就应该警惕了。这意味着你会得到一个Task<Task>>的结果。外部的任务只是在启动内部的任务，因此当外部任务完成时，内部任务才刚开始。在这里，你真正应该使用的是Task.Run，因为它会自动将其解包成一个Task。如果你不能这样做，那么你需要显式地使用Unwrap方法来解包它。 - ServyTask.Run和Task.Factory.StartNew之间的区别，以及为什么在处理异步任务时应该优先选择Task.Run。请注意，翻译过程中不会添加解释或其他信息。 - Stephen Cleary