C#异步任务无限期等待

Question

我正在尝试使用"async"和"await"提供的功能来异步下载网页内容,我遇到了任务等待永远完成的问题.你可以告诉我下面的代码片段有什么问题吗？

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;
}

Answer 1

首先,确保您运行的是.NET 4.5,而不是.NET 4.0.ASP.NET是async在.NET 4.5中生成的.

然后,适当的解决办法是await的结果Task.WhenAll:

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

ASP.NET管道(仅在.NET 4.5中)将检测到您的代码正在运行await并将停止该请求,直到Page_Load运行完成.

在我的博客上解释,Wait在这种情况下同步阻塞任务会导致死锁.