Webclient's DownloadStringCompleted event hand

问题:

I'm trying to make an asynchronous HTTP GET request using Webclient, however, the registered callback never gets called. I've also tried with the sync one, and it worked fine. What am I doing wrong?

WebClient asyncWebRequest;
public AsyncWebRequest(Uri url)
{
    asyncWebRequest = new WebClient();
    url = new Uri("http://www.google.com/");
    // string test = asyncWebRequest.DownloadString(url);  // this works
    asyncWebRequest.DownloadStringCompleted += new DownloadStringCompletedEventHandler(asyncWebRequest_DownloadStringCompleted);
    asyncWebRequest.DownloadStringAsync(url);
}

void asyncWebRequest_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    throw new NotImplementedException();
}

回答1:

Maybe because you disposing the WebClient before it finished downloading. The code execution don't stop on asyncWebRequest.DownloadStringAsync(url); and you are disposing the WebClient object by closing the using statement.

try to dispose the WebClient on asyncWebRequest_DownloadStringCompleted.

results

回答2:

The simpliest solution is to add Console.ReadKey() at the end of AsyncWebRequest(url) method. This way asyncWebRequest.DownloadStringAsync(url) will be able to retrieve data.