从未调用Webclient的DownloadStringCompleted事件处理程序

Question

我正在尝试使用Webclient创建异步HTTP GET请求，但是，注册的回调永远不会被调用。我也试过了同步，它运行正常。我做错了什么？

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();
}

Answer 1

可能是因为您在完成下载之前处置了WebClient。代码执行不会在asyncWebRequest.DownloadStringAsync(url);上停止，并且您通过关闭using语句来处置WebClient对象。

尝试将WebClient置于asyncWebRequest_DownloadStringCompleted。

<强>结果

Answer 2

最简单的解决方案是在Console.ReadKey()方法的末尾添加AsyncWebRequest(url)。这种方式asyncWebRequest.DownloadStringAsync(url)将能够检索数据。