内Task.Run Aync /等待动作（） [英] Aync/Await action within Task.Run()

问题描述

Task.Run（（）=＆GT; {}）放动作代表到队列中，并返回任务。
是否有异步有任何好处/等待Task.Run内（）？
据我所知，Task.Run（），因为如果我们要使用直接等待，然后调用方法将需要进行异步，并会影响调用的地方是必需的。

下面是一个具有异步内Task.Run等待（）样本code。这里提供完整的范例： https：//开头MSDN .microsoft.com / EN-US /库/ hh228607（v = vs.110）的.aspx

  Task.Run（异步（）=＆GT; {等待新的WebClient（）DownloadStringTaskAsync（）;}）;
 

另外，这可能已经完成，

  Task.Run（（）=＆gt;新建的WebClient（）DownloadStringTaskAsync（）结果;。）;
 

由于两个，Task.Run（），并等待将排队工作，并将由线程池来采摘，莫非异步/的Task.Run内的await（）有点多余？

解决方案

  

有异步有任何好处/等待Task.Run（）

之内

您也可以要求相反的问题：为什么你会换一个异步/等待code在Task.Run

？！

这是异步方法返回给调用者只要第一的await被击中（即运行在一个非完成的任务）。所以，如果一个异步方法，首先执行的连胜需要很长的时间 Task.Run 将改变行为：这将导致该方法立即返回并执行第一个连胜关于线程池。

这是在UI场景非常有用，因为这样你可以100％肯定，你是不是阻塞UI。例如：的HttpWebRequest 做DNS解析同步即使您使用的异步方法之一（这基本上是一个库问题/设计错误）。这可以暂停UI线程。所以，你可以使用Task.Run 100％确保UI永远不会阻塞超过几微秒的时间。

所以，回到原来的问题：为什么一个Task.Run身体里面等待？出于同样的原因，你通常等待：解除阻塞线程

。

Task.Run(()=>{}) puts the action delegate into the queue and returns the task . Is there any benefit of having async/await within the Task.Run() ? I understand that Task.Run() is required since if we want to use await directly , then the calling method will need to be made Async and will affect the calling places.

Here is the sample code which has async await within Task.Run() . The full sample is provided here : https://msdn.microsoft.com/en-us/library/hh228607(v=vs.110).aspx

Task.Run(async () => { await new WebClient().DownloadStringTaskAsync("");});

Alternatively this could have been done ,

Task.Run(() => new WebClient().DownloadStringTaskAsync("").Result;);

Since both , Task.Run() and Await will queue the work and will be picked by the thread pool , Could the async/ await within the Task.Run() be a bit redundant ?

解决方案

Is there any benefit of having async/await within the Task.Run()

You also could ask the opposite question: Why would you wrap an async/await code in Task.Run?!

An async method returns to the caller as soon as the first await is hit (that operates on a non-completed task). So if that first execution "streak" of an async method takes a long time Task.Run will alter behavior: It will cause the method to immediately return and execute that first "streak" on the thread-pool.

This is useful in UI scenarios because that way you can make 100% sure that you are not blocking the UI. Example: HttpWebRequestdoes DNS resolution synchronously even when you use one of the async methods (this is basically a library bug/design error). This can pause the UI thread. So you can use Task.Run to be 100% sure that the UI is never blocked for longer than a few microseconds.

So back to the original question: Why await inside a Task.Run body? For the same reason you normally await: To unblock the thread.

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