Question

我一直在使用位于here的.NET Framework并行编程示例，并学习一些非常棒的东西。

具体来说，我正在尝试构建一个简单的程序，该程序会引入一个rss源列表，然后解析出特定的信息。

在解决方案 AsyncDownload_CSharp 中，它提供了许多如何异步下载的示例。我对以下代码有了相当不错的理解：

// Concurrent async requests
Task.Factory.ContinueWhenAll(new []
{
    Download("http://blogs.msdn.com/pfxteam"),
    Download("http://blogs.msdn.com/nativeconcurrency"),
    Download("http://exampleexampleexample.com"), // will fail
    Download("http://msdn.com/concurrency"),
    Download("http://bing.com")
}, ConcurrentTasksCompleted).Wait();

上面的代码有任务硬编码列表，我一直试图弄清楚如何动态生成任务列表，我在运行时通过foreach循环或其他一些方式分配url的值重复循环。

我想出的代码是动态构建任务列表并将其传递给Task.Factory.ContinueWhenAll看起来像这样：

private void btnFeeds_Click(object sender, EventArgs e)
{
    string[] urls = new[] { "http://blogs.msdn.com/pfxteam", "http://msdn.com/concurrency", "http://bing.com" };
    List<Task<string>> tasks = new List<Task<string>>();

    foreach (string url in urls)
    {
        tasks.Add(Download(url));
    }

    Task.Factory.ContinueWhenAll(tasks, ConcurrentTasksCompleted).Wait();

}

不幸的是，当我单步执行调试器时，我不仅看到正在构建的任务列表，而且因为它们被添加到任务列表中，但我也看到它们在传递给Task.Factory.ContinueWhenAll之前被执行。

我认为我需要使用Lambda表达式而不是foreach循环将每个表达式传递给Task.Factory.ContinueWhenAll。不幸的是，我很难弄清楚如何使用Lambda表达式应用任务的URL。

我不确定我尝试做的事情是否可行，所以任何建议，指示或帮助都会受到高度赞赏。

Answer 1

因为您正在使用方法DownLoad(url)来创建任务对象。在DownLoad(url)方法中，任务实际上已经启动，您可以在ParallelExtensionsExtras / Extensions / EAP / WebClientExtensions.cs中检查代码

 /// <summary>Downloads the resource with the specified URI as a byte array, asynchronously.</summary> 
    /// <param name="webClient">The WebClient.</param> 
    /// <param name="address">The URI from which to download data.</param> 
    /// <returns>A Task that contains the downloaded data.</returns> 
    public static Task<byte[]> DownloadDataTask(this WebClient webClient, string address) 
    { 
        return DownloadDataTask(webClient, new Uri(address)); 
    } 

    /// <summary>Downloads the resource with the specified URI as a byte array, asynchronously.</summary> 
    /// <param name="webClient">The WebClient.</param> 
    /// <param name="address">The URI from which to download data.</param> 
    /// <returns>A Task that contains the downloaded data.</returns> 
    public static Task<byte[]> DownloadDataTask(this WebClient webClient, Uri address) 
    { 
        // Create the task to be returned 
        var tcs = new TaskCompletionSource<byte[]>(address); 

        // Setup the callback event handler 
        DownloadDataCompletedEventHandler handler = null; 
        handler = (sender, e) => EAPCommon.HandleCompletion(tcs, e, () => e.Result, () => webClient.DownloadDataCompleted -= handler); 
        webClient.DownloadDataCompleted += handler; 

        // Start the async work 
        try 
        { 
            webClient.DownloadDataAsync(address, tcs); 
        } 
        catch(Exception exc) 
        { 
            // If something goes wrong kicking off the async work, 
            // unregister the callback and cancel the created task 
            webClient.DownloadDataCompleted -= handler; 
            tcs.TrySetException(exc); 
        } 

        // Return the task that represents the async operation 
        return tcs.Task; 
    }

动态地将任务分配给Task.Factory.ContinueWhenAll要处理的Task数组

1 个答案: