使用WebClient异步下载文件不起作用

本文关键字：文件 不起作用 下载 异步 WebClient 使用 | 更新日期: 2023-09-27 18:08:43

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!");
        }
    }
}

下面是我的main:

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。

使用WebClient异步下载文件不起作用

由于Async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。这将在文件下载完成之前终止程序执行。

你需要阻塞线程直到文件下载完成。你考虑过使用。net 4.5 (https://msdn.microsoft.com/en-us/library/hh191443.aspx)中的async/await吗?你也可以使用Progress/Completed事件来同步线程，这样在文件完成之前应用程序不会退出。Console.ReadLine()也应该阻塞当前线程，直到按下Enter键。

我扩展了你的代码来支持线程阻塞。这将使主线程在两次检查之间休眠15秒，直到文件完成。我使用关键字"volatile"(https://msdn.microsoft.com/en-us/library/x13ttww7.aspx)来确保总是抓取最新的值。这对于多线程非常重要。_completed = true位于if/else语句的外部，因为我们希望它即使被取消也能退出。如果需要，可以扩展此解决方案来处理取消的下载。 
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);
    }
}
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