使用 WebClient 异步下载文件不起作用

Question

我正在使用 C# 在 VS15 中制作控制台应用程序。 这是我的下载类：

class DownloadGamefile
{
    public  void DownloadFile(string address, string location)
    {
        WebClient client = new WebClient();
        Uri Uri = new Uri(address);

        client.DownloadFileCompleted += new AsyncCompletedEventHandler(Completed);

        client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgress);
        client.DownloadFileAsync(Uri, location);

    }

    private void DownloadProgress(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 void Completed(object sender, AsyncCompletedEventArgs e)
    {
        if (e.Cancelled == true)
        {
            Console.WriteLine("Download has been canceled.");
        }
        else
        {
            Console.WriteLine("Download completed!");
        }
    }
}

这是我的主要内容：

class Program
{
    static void Main(string[] args)
    {

        DownloadGamefile DGF = new DownloadGamefile();

        DGF.DownloadFile("URL", @"C:\Users\LocDaiLe\Desktop\file.file");

    }
}

文件出现在正确的文件夹中，但大小为 0 字节，我的控制台没有显示任何下载进度。

Answer 1

由于异步调用不会阻塞当前线程，我的猜测是 DownloadFileAsync (https://msdn.microsoft.com/en-us/library/ms144196(v=vs.110).aspx) 立即返回，导致您的 DownloadFile 也立即返回。这将在文件完成下载之前结束程序执行。

您需要阻塞线程，直到文件完成下载。您是否考虑过使用 .NET 4.5 (https://msdn.microsoft.com/en-us/library/hh191443.aspx) 中的 async/await？您还可以使用 Progress/Completed 事件来同步您的线程，以便应用程序在文件完成之前不会退出。 Console.ReadLine() 也应该阻塞当前线程，直到按下 Enter。

我扩展了您的代码以支持线程阻塞。这将使主线程在检查之间休眠 1 秒，直到文件完成。我使用关键字“volatile”（https://msdn.microsoft.com/en-us/library/x13ttww7.aspx）来确保始终获取最新的值。这对于多线程很重要。 _completed = true 位于 if/else 之外，因为我们希望它即使被取消也能退出。如果需要，您可以扩展此解决方案以处理取消的下载。

class DownloadGamefile
{
    private volatile bool _completed;

    public void DownloadFile(string address, string location)
    {
        WebClient client = new WebClient();
        Uri Uri = new Uri(address);
        _completed = false;

        client.DownloadFileCompleted += new AsyncCompletedEventHandler(Completed);

        client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgress);
        client.DownloadFileAsync(Uri, location);

    }

    public bool DownloadCompleted { get { return _completed; } }

    private void DownloadProgress(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 void Completed(object sender, AsyncCompletedEventArgs e)
    {
        if (e.Cancelled == true)
        {
            Console.WriteLine("Download has been canceled.");
        }
        else
        {
            Console.WriteLine("Download completed!");
        }

        _completed = true;
    }
}

class Program
{
    static void Main(string[] args)
    {

        DownloadGamefile DGF = new DownloadGamefile();

        DGF.DownloadFile("URL", @"C:\Users\LocDaiLe\Desktop\file.file");

        while (!DGF.DownloadCompleted)
            Thread.Sleep(1000);
    }
}