等待上一个完成的任务一样task.Result？ [英] Await on a completed task same as task.Result?

问题描述

我目前正在读，在C＃食谱并发由斯蒂芬·克利里，我注意到了以下技术：

I'm currently reading "Concurrency in C# Cookbook" by Stephen Cleary, and I noticed the following technique:

var completedTask = await Task.WhenAny(downloadTask, timeoutTask);  
if (completedTask == timeoutTask)  
  return null;  
return await downloadTask;  

的 downloadTask 的是httpclient.GetStringAsync一个电话，和 timeoutTask 的正在执行Task.Delay。

downloadTask is a call to httpclient.GetStringAsync, and timeoutTask is executing Task.Delay.

在的情况下，它没有超时，那么downloadTask已经完成。为什么因为任务已经完成，需要做第二次的await而不是返回downloadTask.Result的，？

In the event that it didn't timeout, then downloadTask is already completed. Why is necessary to do a second await instead of returning downloadTask.Result, given that the task is already completed?

推荐答案

目前已经有一些很好的答案/评论在这里，但只是为了附和...

There are already some good answers/comments here, but just to chime in...

有两个原因我preFER 等待在结果（或等待）。首先，错误处理是不同的; 等待不换行除外在 AggregateException 。理想情况下，异步code不应该有应对 AggregateException 可言，除非它专门的希望的到。

There are two reasons why I prefer await over Result (or Wait). The first is that the error handling is different; await does not wrap the exception in an AggregateException. Ideally, asynchronous code should never have to deal with AggregateException at all, unless it specifically wants to.

第二个原因是多了几分含蓄。正如我形容我的博客（在书中），<一个href=\"http://blog.stephencleary.com/2012/07/dont-block-on-async-$c$c.html\"><$c$c>Result/<$c$c>Wait可能导致死锁和甚至会引起更微妙的僵局在异步方法使用时。所以，当我阅读到code和我看到了一个结果或等待，这是一个直接的警告旗。如果你的的结果 / 等待是唯一正确的绝对肯定的那个任务已经完成。这不仅是很难一眼看出（在现实世界code），但它也更脆至code的变化。

The second reason is a little more subtle. As I describe on my blog (and in the book), Result/Wait can cause deadlocks, and can cause even more subtle deadlocks when used in an async method. So, when I'm reading through code and I see a Result or Wait, that's an immediate warning flag. The Result/Wait is only correct if you're absolutely sure that the task is already completed. Not only is this hard to see at a glance (in real-world code), but it's also more brittle to code changes.

这并不是说，结果 / 等待应的从不的使用。我遵循这些原则在我自己的code：

That's not to say that Result/Wait should never be used. I follow these guidelines in my own code:

在应用程序
1. 异步code只能使用的await 。


2. 异步工具code（在库）可以偶尔使用结果 / 等待如果code真的会需要它。这种用法大概应该有意见。


3. 的并行的任务code可以使用结果和等待。

1. Asynchronous code in an application can only use await.
2. Asynchronous utility code (in a library) can occasionally use Result/Wait if the code really calls for it. Such usage should probably have comments.
3. Parallel task code can use Result and Wait.

请注意：（1）是目前常见的情况，因此我倾向于使用等待无处不在，对待其他案件作为例外的一般规则。

Note that (1) is by far the common case, hence my tendency to use await everywhere and treat the other cases as exceptions to the general rule.

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