何时使用OrderByCompletion（乔恩斯基特）VS Parallel.ForEach与异步委托 [英] When to use OrderByCompletion (Jon Skeet) vs Parallel.ForEach with async delegates

问题描述

近日乔恩斯基特在NDC伦敦谈到了C＃5异步/等待和presented 按完成订购的一的异步任务列表的想法。链接<一个href=\"http://msmvps.com/blogs/jon_skeet/archive/2012/01/16/eduasync-part-19-ordering-by-completion-ahead-of-time.aspx\" rel=\"nofollow\">http://msmvps.com/blogs/jon_skeet/archive/2012/01/16/eduasync-part-19-ordering-by-completion-ahead-of-time.aspx

我有点困惑或者我应该说我不知道​​什么时候这个技术更适合使用。

我不明白这一点，下面的例子之间的差异

  VAR包=新ConcurrentBag＆LT;对象＆gt;（）;
Parallel.ForEach（MyCollection的，异步项=＆GT;
{
  //一些pre的东西
  VAR响应=等待的GetData（项目）;
  bag.Add（响应）;
  //一些东西后
}
 

或的 ForEachAsync 的：由斯蒂芬Toub解释 - 的 http://blogs.msdn.com/b/pfxteam/archive/2012/03/05/10278165.aspx

  

编辑：发现了的博客文章从斯蒂芬Toub 解释＆LT; =>处理任务，他们完成。值得一读。在阅读完之后，我可以清楚地了解它是如何工作的原因，还当使用这种技术。

解决方案

• 不要使用 Parallel.ForEach 执行异步 code。 Parallel.ForEach 不明白异步，所以你的拉姆达将变成异步无效，这将无法正常工作（ Parallel.ForEach 将所有的工作完成之前返回;异常将无法得到妥善处理;其他可能的问题）。




• 使用类似 ForEachAsync（）当你有对象的集合（不是工作 S） ，要执行的为他们每个人的某些异步动作和行动应并行执行。




• 使用 OrderByCompletion（）当你有工作 S，你要执行一些动作的集合（异步与否）为每个结果工作，动作应的不的并行执行，并且希望基于顺序执行的行动其中，工作必须填写。

Recently Jon Skeet at NDC London spoke about C# 5 async/await and presented the idea of "ordering by completion" a list of async tasks. A link http://msmvps.com/blogs/jon_skeet/archive/2012/01/16/eduasync-part-19-ordering-by-completion-ahead-of-time.aspx

I am a bit confused or should I say I am not sure when will this technique be more appropriate to use.

I cannot understand the difference between this and the below example

var bag = new ConcurrentBag<object>();
Parallel.ForEach(myCollection, async item =>
{
  // some pre stuff
  var response = await GetData(item);
  bag.Add(response);
  // some post stuff
}

or ForEachAsync as explained by Stephen Toub - http://blogs.msdn.com/b/pfxteam/archive/2012/03/05/10278165.aspx

EDIT: Found a blog post from Stephen Toub explaining "Ordering by completion" <=> "Processing tasks as they complete". Worth reading. After reading this I could clearly understand the reasons how it works and also when to use this technique.

解决方案

• Don't use Parallel.ForEach to execute async code. Parallel.ForEach doesn't understand async, so your lambda will be turned into async void, which won't work correctly (Parallel.ForEach will return before all work is done; exceptions won't be handled properly; possibly other issues).

• Use something like ForEachAsync() when you have a collection of objects (not Tasks), you want to perform some async action for each of them and the actions should execute in parallel.

• Use OrderByCompletion() when you have a collection of Tasks, you want perform some action (asynchronous or not) for the result of each Task, the actions should not execute in parallel and you want to execute the actions based on the order in which the Tasks complete.

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