多个WebClient无法正常工作? [英] Multiple WebClients not working?

问题描述

我正在尝试使用三个单独的WebClient下载三个文件.我用这个:

I am trying to download three files with three separate WebClients. I use this:

    void client1_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
    {
        MessageBox.Show("CLIENT1");
    }

    void client2_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
    {
        MessageBox.Show("CLIENT2");
    }

    void client3_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
    {
        MessageBox.Show("CLIENT3");
    }

    private void mwindow_Loaded(object sender, RoutedEventArgs e)
    {
        string rand = new Random().Next().ToString();
        WebClient client1 = new WebClient();
        client1.OpenReadCompleted += client1_OpenReadCompleted;
        client1.OpenReadAsync(new Uri("http://www.example.com/file.exe?r=" + rand));

        WebClient client2 = new WebClient();
        client2.OpenReadCompleted += client2_OpenReadCompleted;
        client2.OpenReadAsync(new Uri("http://www.example.com/file.exe?r=" + rand));

        WebClient client3 = new WebClient();
        client3.OpenReadCompleted += client3_OpenReadCompleted;
        client3.OpenReadAsync(new Uri("http://www.example.com/file.exe?r=" + rand));
    }

使用此功能时，无论我做什么，三个WebClient中只有两个会完成.使用此代码，我得到两个消息框"CLIENT1"和"CLIENT2"，但是"CLIENT3"从不出现.没有引发异常或任何异常.没发生什么事.如果我颠倒了WebClient的顺序，则 client3 和 client2 起作用，但 client1 无效.我尝试更改代码以使WebClient一次下载一个，而不是异步下载:

When using this, no matter what I do, only two out of the three WebClients will finish. Using this code, I get two message boxes "CLIENT1", and "CLIENT2", but "CLIENT3" never appears. No exceptions are thrown or anything. Nothing happens. If I reverse the order of the WebClients, client3 and client2 work but not client1. I have tried changing the code to make the WebClients download one at a time instead of asynchronously:

        WebClient client1 = new WebClient();
        Stream str1 = client1.OpenRead(new Uri("http://www.example.com/file.exe?r=" + rand));
        MessageBox.Show("CLIENT1");

        WebClient client2 = new WebClient();
        Stream str2 = client2.OpenRead(new Uri("http://www.example.com/file.exe?r=" + rand));
        MessageBox.Show("CLIENT2");

        WebClient client3 = new WebClient();
        Stream str3 = client3.OpenRead(new Uri("http://www.example.com/file.exe?r=" + rand));
        MessageBox.Show("CLIENT3");

但是，程序冻结在 Stream str3 = client3.OpenRead(new Uri("http://www.example.com/file.exe?r=" + rand)); 线. client1 同步下载所有文件而不是创建多个WebClient也会冻结第三个文件.

However, the program freezes on the Stream str3 = client3.OpenRead(new Uri("http://www.example.com/file.exe?r=" + rand)); line. Having client1 download all the files synchronously instead of creating multiple WebClients also freezes on the third file.

推荐答案

我认为您遇到的是两个问题的结合.第一个是默认情况下，并发 WebRequest 连接的数量限制为2.您可以通过创建从 WebClient 派生的类并覆盖 GetWebRequest 方法来更改此方法，如下所示:

I think what you're experiencing is a combination of two issues. The first one being that the number of concurrent WebRequest connections is limited to 2 by default. You can change that by creating a class derived from WebClient and overriding the GetWebRequest method like so:

public class ExtendedWebClient : WebClient
{
    /// <summary>
    /// Gets or sets the maximum number of concurrent connections (default is 2).
    /// </summary>
    public int ConnectionLimit { get; set; }

    /// <summary>
    /// Creates a new instance of ExtendedWebClient.
    /// </summary>
    public ExtendedWebClient()
    {
        this.ConnectionLimit = 2;
    }

    /// <summary>
    /// Creates the request for this client and sets connection defaults.
    /// </summary>
    protected override WebRequest GetWebRequest(Uri address)
    {
        var request = base.GetWebRequest(address) as HttpWebRequest;

        if (request != null)
        {
            request.ServicePoint.ConnectionLimit = this.ConnectionLimit;
        }

        return request;
    }
}

我看到的第二个问题是，您没有关闭/处理调用 OpenRead 时返回的 Stream ，因此这两个请求似乎没有完成，直到垃圾收集器决定为您打开并关闭这些流为止.

The second problem I see is that you're not closing/disposing of the Stream returned when you call OpenRead, so it would appear that the two requests do not complete until such time when the Garbage Collector decides to kick in and close those streams for you.

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