使用Parallel.ForEach与/或异步/的await [英] using Parallel.ForEach with/or async/await

问题描述

我尝试验证我的形象的网址，看看他们是否是有效的。我有这么多的人，它需要几个小时才能完成这个任务。因此，我决定以异步方式做到这一点。我想知道，如果有如下当仁不让code的任何大的差异或优势。

I try to verify my image URLs to see whether or not they are valid. I have so many of them that it take hours to complete this task. Therefore, I decided to do it asynchronously. I would like to know if there are any big differences or advantage of doing my code as below.

我的主要职责是：

Async Function testUrl_async(ByVal myImageurl As String) As Task(Of Boolean)

   myHttpResponse = Await myHttpClient.GetAsync(myImageurl)
    If myHttpResponse.IsSuccessStatusCode Then
        mySuccess = True
    Else
        mySuccess = False
    End If

    Return mySuccess
End Function

 Function testUrl(ByVal myImageurl As String) As  Boolean

   myHttpResponse = myHttpClient.GetAsync(myImageurl)
    If myHttpResponse.IsSuccessStatusCode Then
        mySuccess = True
    Else
        mySuccess = False
    End If

    Return mySuccess
End Function

1）采用异步等待。

1) using async await.

For Each myImage In myImages
    Dim result=await testUrl_async(myImageUrl).Result 
    'some code                  
Next

2）使用并行的foreach

2) using parallel foreach

Parallel.ForEach(myImages, 
    Sub(myImage)
        testUrl(pictureComponent.websiteShop.hqpatronen, myImageUrl) 
        'some code
    End Sub)

3）使用并行的foreach和asnyc /的await

3) using parallel foreach and asnyc/await

Parallel.ForEach(myImages, 
    Sub(myImage)
        await testUrl_async(pictureComponent.websiteShop.hqpatronen, myImageUrl) 
    'some code
    End Sub)

第三个可能是最好的解决办法，但它不会允许我这样称呼等待 / 异步中在的ForEach 。

如果我用的是第二个， testurl 函数具有异步HTTP调用，但不能与等待， thereofore它与异常消息崩溃：

If I use the second one, the testurl function has the async http call, but not with Await, thereofore it crashes with the exception message:

[TaskCanceledException：一个任务被取消]

[TaskCanceledException: A task was canceled.]

在调用 myHttpClient.GetAsync 行。我猜测它抛出这个异常，因为的ForEach 已经结束，并要求取消HttpClient的，但并没有完成它的工作呢。我该如何处理这一点，如果这可能是最好的解决办法？

on the line that calls myHttpClient.GetAsync. I am guessing that it throws this exception because ForEach has ended and cancellation was requested but httpclient didn't finish its job yet. How can i handle this if this could be the best solution?

或者任何其他的解决方案，它使我的工作速度更快。

Alternatively any other solution which makes my job faster.

推荐答案

您肯定不希望使用 Parallel.ForEach 。 并行是在多个内核US preading CPU绑定的算法，这将提供你任何好处（在你的情况下，你的算法是不是CPU绑定）。

You certainly don't want to use Parallel.ForEach. Parallel is for spreading CPU-bound algorithms over multiple cores, which would provide you no benefit (in your scenario, your algorithm is not CPU-bound).

你真正想要的是并发性，而不是并行。异步并行是可以做到用 Task.WhenAll ：

What you actually want is concurrency, not parallelism. Asynchronous concurrency can be done using Task.WhenAll:

Dim tasks = myImages.Select(Function(x) testUrl_async(x))
Dim results = Await Task.WhenAll(tasks)

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