C# async tasks waiting indefinitely

问题

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;
}

回答1:

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.

回答2:

+1 Stephen Cleary. Just came to know you need to have async before void type with Page_Load as given below:

protected async void Page_Load(object sender, EventArgs e)
{
   var tasks = websites.Select(GenerateSomeContent);
   await Task.WhenAll(tasks);
}

And then in your code-behind file (in case asp.net web form app) should also have Async="true" attribute.

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Async="true" Inherits="EmptyWebForm._default" %>

Hope this helps visitors.

来源：https://stackoverflow.com/questions/14150247/c-sharp-async-tasks-waiting-indefinitely

标签

ASP.NET

async-await

dotnet-httpclient