Question

我正在尝试从用户提供的网址获取标头，并且所有内容似乎都正常工作，直到我尝试http://www.google.com，这给了我以下异常：

System.NullReferenceException未处理Message = NotSupportedException 堆栈跟踪：在System.Net.Browser.OHWRAsyncResult.get_AsyncWaitHandle（）在System.Net.Browser.AsyncHelper.BeginOnUI（SendOrPostCallback beginMethod，Object state）在System.Net.Browser.ClientHttpWebRequest.EndGetResponse（IAsyncResult asyncResult）在Network_Monitor.Checks.URLCheckResults.RespCallback（IAsyncResult asynchronousResult）在System.Net.Browser.ClientHttpWebRequest。＆lt;＆gt; c__DisplayClassa.b__8（对象state2）在System.Threading.ThreadPool.WorkItem.WaitCallback_Context（对象状态）在System.Threading.ExecutionContext.Run（ExecutionContext executionContext，ContextCallback回调，对象状态）在System.Threading.ThreadPool.WorkItem.doWork（Object o）在System.Threading.Timer.ring（）

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

我用谷歌搜索了几个小时寻找解决方案，但现有的解决方案都没有。导致此问题出现在Google.com而非其他网站上的原因是什么？

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);
        }
    }
}

例外情况发生在using (HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult))行，并带有以下屏幕截图（右键单击并单击查看图像以查看更大）：

Exception Image

Answer 1

我：它完全使用yahoo.com，google.com和   msn.com。我和你的代码之间的唯一区别是我跑了   Dispatcher之外的迭代。谷歌有任何机会打嗝   当你试过？
     加布里埃尔：我不知道该说些什么。创建它作为答案，我会   奖励你+100声望。

NullReferenceException未处理HttpWebResponse

1 个答案: