Webclient's DownloadStringCompleted event handler never called

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.

来源：https://stackoverflow.com/questions/7373172/webclients-downloadstringcompleted-event-handler-never-called