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我正在尝试从用户提供的 url 获取标头，并且在我尝试之前一切似乎都工作正常，http://www.google.com这给了我以下异常：

System.NullReferenceException 未处理 Message=NotSupportedException StackTrace: at System.Net.Browser.OHWRAsyncResult.get_AsyncWaitHandle() at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state) at System.Net.Browser.ClientHttpWebRequest.EndGetResponse( IAsyncResult asyncResult) 在 System.Net.Browser.ClientHttpWebRequest 的 Network_Monitor.Checks.URLCheckResults.RespCallback(IAsyncResult asynchronousResult)。<>c__DisplayClassa.b__8(Object state2) 在 System.Threading.ThreadPool.WorkItem.WaitCallback_Context(Object state) 在 System。 Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) 在 System.Threading.ThreadPool.WorkItem.doWork(Object o) 在 System.Threading.Timer。戒指（）

调用堆栈：

System.Windows.dll!System.Net.Browser.AsyncHelper.BeginOnUI(System.Threading.SendOrPostCallback beginMethod, object state) + 0xc3 bytes 
System.Windows.dll!System.Net.Browser.ClientHttpWebRequest.EndGetResponse(System.IAsyncResult asyncResult) + 0x41 bytes 
Network Monitor.dll!Network_Monitor.Checks.URLCheckResults.RespCallback(System.IAsyncResult asynchronousResult) Line 55 + 0x3 bytes C#
System.Windows.dll!System.Net.Browser.ClientHttpWebRequest.InvokeGetResponseCallback.AnonymousMethod__8(object state2) + 0x1b bytes 
mscorlib.dll!System.Threading.ThreadPool.WorkItem.WaitCallback_Context(object state) + 0x18 bytes   
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x63 bytes    
mscorlib.dll!System.Threading.ThreadPool.WorkItem.doWork(object o) + 0x47 bytes 
mscorlib.dll!System.Threading.Timer.ring() + 0x70 bytes

我已经用谷歌搜索了几个小时来寻找解决方案，但现有的解决方案都不起作用。是什么导致它出现在 Google.com 而不是其他网站？

public void CheckURL(String URL)
{
    lblResults.Text = "Checking...";

    var wr = HttpWebRequest.Create(URL);
    wr.Method = "HEAD";

    IAsyncResult asyncResult = (IAsyncResult)wr.BeginGetResponse(RespCallback, wr);
}

private void RespCallback(IAsyncResult asynchronousResult)
{
    try
    {
        HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;
        using (HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult))
        {
            Dispatcher.BeginInvoke(() =>
                {
                    for (int i = 0; i < response.Headers.Count; ++i)
                    {
                        if (!lblResults.Text.Equals("Checking..."))
                        {
                            lblResults.Text += "\n";
                        }
                        else
                        {
                            lblResults.Text = "";
                        }

                        lblResults.Text += "Header Name:" + response.Headers.AllKeys[i] + ", Header value :" + response.Headers[response.Headers.AllKeys[i]];
                    }
                });
        }
    }
    catch (WebException ex)
    {
        if (((HttpWebResponse)ex.Response).StatusCode == HttpStatusCode.NotFound)
        {
            Dispatcher.BeginInvoke(() =>
            {
                lblResults.Text = "Page Not Found (404)";
            });
        }
        else
        {
            MessageBox.Show(ex.Message, "Program Name", MessageBoxButton.OK);
        }
    }
}

异常发生在using (HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult))以下屏幕截图（右键单击并单击查看图像以查看大图）：

异常图像

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1 回答 1

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ME: It just ran for me perfectly using yahoo.com, google.com, and msn.com. The only difference between mine and your code was I ran the iteration outside of the Dispatcher. Any chance Google was hiccuping when you tried?

Gabriel: I don't know what to say. Create that as an answer and I'll reward you the +100 reputation.

于 2012-08-14T20:42:30.357 回答