异步WebClient不是真正的异步？

Question

我在类中创建了一个异步WebClient请求,如下所示:

public class Downstream
    {
        public bool StartDownstream()
        {
            WebClient client = new WebClient();

            client.Headers.Add("user-agent", "Mozilla/4.0 [...]");
            client.Headers.Add("Content-Type","application/x-www-form-urlencoded");
            try
            {

                byte[] postArray = Encoding.UTF8.GetBytes("somevar=foo&someothervar=bar");
                Uri uri = new Uri("http://www.examplesite.com/somepage.php");

                client.UploadDataCompleted += 
                new UploadDataCompletedEventHandler(client_UploadDataCompleted);
                client.UploadDataAsync(uri, postArray);
            }
            catch (WebException e)
            {
                MessageBox.Show("A regular Web Exception");
            }
            catch (NotSupportedException ne)
            {
                MessageBox.Show("A super Web Exception");
            }
            return true;
        }

        void client_UploadDataCompleted(object sender, UploadDataCompletedEventArgs e)
        {
            MessageBox.Show("The WebClient request completed");
        }
    }

然后我创建一个新的类实例并在此处运行该方法:

Downstream Downstream1 = new Downstream();
Downstream1.StartDownstream();

当我这样做时,表单运行的线程似乎挂起,直到WebClient获得响应.为什么是这样？我已经使用过该UploadDataAsync方法,所以它不应该是异步的吗？

编辑:

这是我的调用堆栈:

    [External Code] 
>   Arcturus.exe!Arcturus.Downstream.StartDownstream() Line 36 + 0x18 bytes C#
    Arcturus.exe!Arcturus.MainWindow.btnLogin_Click(object sender, System.Windows.RoutedEventArgs e) Line 111 + 0x12 bytes  C#
    [External Code]

这就是我运行我的应用程序时发生的一切,只是挂在StartDownstream()和client.UploadDataAsync(uri, postArray);方法上.

Answer 1

我遇到的问题是 WebClient 使用与 UI 相同的线程，无论它是否异步。我决定改用BackgroundWorker。

感谢您的回答！