C＃异步任务无限期等待 [英] C# async tasks waiting indefinitely

问题描述

我试图使用由异步＆放大器提供的功能; 等待异步下载网页内容，我必须到那里的任务在等待永远完成的问题。能否请你让我知道什么是错用下面的code段？

 保护无效的Page_Load（对象发件人，EventArgs的发送）
{
    VAR网站=新的String [] {http://www.cnn.com，http://www.foxnews.com};
    变种任务= websites.Select（GenerateSomeContent）.ToList（）;    //我不想用'等待Tasks.WhenAll（任务）因为我需要阻止
    //页面加载，直到所有的网页内容下载
    Task.WhenAll（任务）.Wait（）;    //此行是从来没有打在调试
    VAR someValue中=完成;
}静态异步任务＆lt;结果＆GT; GenerateSomeContent（字符串URL）
{
    VAR的客户=新的HttpClient（）;
    VAR响应=等待client.GetAsync（URL）; //指日可待响应
    VAR内容=等待response.Content.ReadAsStringAsync（）;
    无功输出=新的结果{内容=内容};
    返回输出;
}// Sample类来保存结果
公共类结果
{
    公共字符串的内容;
}
 

解决方案

首先，确保你在.NET 4.5上运行，而不是.NET 4.0。 ASP.NET被做了异步 -Aware在.NET 4.5。

然后，适当的解决办法就是等待的结果 Task.WhenAll ：

  VAR任务= websites.Select（GenerateSomeContent）;
等待Task.WhenAll（任务）;
 

ASP.NET管道（在.NET 4.5只），将检测到您的code是等待和荷兰国际集团将停止，直到的Page_Load 运行结束。

同步方式使用等待在这种情况下导致死锁的我在我的博客解释。

I am trying to use the functionality provided by "async" & "await" to asynchronously download webpage content and I have into issues where the Tasks are waiting forever to complete. Could you please let me know what is wrong with the following code snippet?

protected void Page_Load(object sender, EventArgs e)
{
    var websites = new string[] {"http://www.cnn.com","http://www.foxnews.com"};
    var tasks = websites.Select(GenerateSomeContent).ToList();

    //I don't want to use 'await Tasks.WhenAll(tasks)' as I need to block the
    //page load until the all the webpage contents are downloaded
    Task.WhenAll(tasks).Wait();

    //This line is never hit on debugging
    var somevalue = "Complete";
}

static async Task<Results> GenerateSomeContent(string url)
{
    var client = new HttpClient();
    var response = await client.GetAsync(url); //Await for response
    var content = await response.Content.ReadAsStringAsync();
    var output = new Results {Content = content};
    return output;
}

//Sample class to hold results
public class Results
{
    public string Content;
}

解决方案

First, make sure you're running on .NET 4.5, not .NET 4.0. ASP.NET was made async-aware in .NET 4.5.

Then, the proper solution is to await the result of Task.WhenAll:

var tasks = websites.Select(GenerateSomeContent);
await Task.WhenAll(tasks);

The ASP.NET pipeline (in .NET 4.5 only) will detect that your code is awaiting and will stall that request until Page_Load runs to completion.

Synchronously blocking on a task using Wait in this situation causes a deadlock as I explain on my blog.

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