webclient.openfileasync fire downloadProgressChanged

根据

http://msdn.microsoft.com/en-us/library/system.net.webclient.downloadprogresschanged.aspx ,

OpenFileAsync应该让DownloadProgressChanged在它取得进展时触发。

我根本不能让它着火。但是,使用DownloadDataAsync和DownloadFileAsync可以激发。

下面是一个简单的例子:

using System;
using System.Net;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            WebClient client = new WebClient();
            client.OpenReadCompleted += new OpenReadCompletedEventHandler(client_OpenReadCompleted);
            client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
            client.OpenReadAsync(new Uri("http://www.stackoverflow.com"));
            Console.ReadKey();
        }

        static void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            Console.WriteLine("{0}% Downloaded", e.ProgressPercentage);
        }

        static void client_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
        {
            Console.WriteLine("Open Read Completed");
        }
    }
}

对于我来说,DownloadProgressChanged事件永远不会触发,尽管它更改为DownloadFileAsync或DownloadDataAsync,并且确实如此。