DownloadFileAsync不下载文件

Question

我试图编写一个简单的应用程序，从网上下载一个文件。

class Program
{
    static void Main(string[] args)
    {
        WebClient client = new WebClient();
        Uri uri = new Uri("http://download.thinkbroadband.com/100MB.zip");

        // Specify that the DownloadFileCallback method gets called
        // when the download completes.
        client.DownloadFileCompleted += new AsyncCompletedEventHandler(DownloadFileCallback2);
        // Specify a progress notification handler.
        client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressCallback);
        client.DownloadFileAsync(uri, "serverdata.txt");

        Console.WriteLine("Download successful.");

    }
    private static void DownloadProgressCallback(object sender, DownloadProgressChangedEventArgs e)
    {
        // Displays the operation identifier, and the transfer progress.
        Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...",
            (string)e.UserState,
            e.BytesReceived,
            e.TotalBytesToReceive,
            e.ProgressPercentage);
    }
    private static void DownloadFileCallback2(object sender, AsyncCompletedEventArgs e)
    {
        // Displays the operation identifier, and the transfer progress.
        Console.WriteLine("Download complete");
    }
}

我在这一行中添加了断点：Console.WriteLine("Download complete");，但它从未命中。程序创建空的serverdata.txt文件。我在控制台中没有收到关于从DownloadProgressCallback下载%的任何更新。我做错了什么？

Answer 1

我还没有尝试过这种方法，但是您可以尝试使用IsBusy属性：

while(client.IsBusy)
     Thread.Sleep(1000);

Console.WriteLine("Download successful.");

或者，如果使用的是WebClient.DownloadFileTaskAsync 4.5，则使用.NET方法

client.DownloadFileTaskAsync(uri, "serverdata.txt").Wait();
Console.WriteLine("Download successful.");

Answer 2

正如其他人所使用的DownloadFileTaskAsync a所示，在等待任务完成时，您的生活会更轻松。您可以异步await结果，也可以调用Wait()来执行阻塞等待。

以下是代码：

private static void DownloadProgressCallback(object sender, DownloadProgressChangedEventArgs e)
    {
        // Displays the operation identifier, and the transfer progress.
        Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...",
            ((TaskCompletionSource<object>)e.UserState).Task.AsyncState,
            e.BytesReceived,
            e.TotalBytesToReceive,
            e.ProgressPercentage);
    }

    static void Main(string[] args)
    {
        WebClient client = new WebClient();
        Uri uri = new Uri("http://download.thinkbroadband.com/100MB.zip");

        // Specify a progress notification handler.
        client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressCallback);
        var task = client.DownloadFileTaskAsync(uri, "serverdata.txt"); // use Task based API

        task.Wait(); // Wait for download to complete, can deadlock in GUI apps

        Console.WriteLine("Download complete");

        Console.WriteLine("Download successful.");
    }

Wait()调用可以在基于GUI的应用程序上死锁，但这对您的情况很好。