Question

我正在尝试使用三个单独的WebClient下载三个文件。我用这个：

    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));
    }

使用它时，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“CLIENT3”永远不会出现。不会抛出异常或任何异常。什么都没发生。如果我颠倒了WebClient的顺序，client3和client2可以正常工作，但不能client1。我尝试更改代码，使WebClient一次下载一个而不是异步下载：

        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");

然而，程序在Stream str3 = client3.OpenRead(new Uri("http://www.example.com/file.exe?r=" + rand));行冻结。 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。

Answer 1

我认为您遇到的是两个问题的组合。第一个是默认情况下并发WebRequest连接数限制为2。您可以通过创建一个派生自WebClient的类并重写GetWebRequest方法来改变它：

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，当您拨打Stream时，您没有关闭/处理返回的OpenRead，因此看起来这两个请求直到垃圾收集器决定为您启动并关闭这些流。

多个WebClient无法正常工作？

1 个答案: