Question

我正在编写一个背景音频代理，可以播放来自在线流的音乐，还会定期检查曲目名称和艺术家的更新。我正在尝试使用HttpWebRequest对象来获取名称和艺术家，但每当我调用HttpWebResponse trackResponse = (HttpWebResponse)trackRequest.EndGetResponse(result);时，都会抛出以下错误。

A first chance exception of type 'System.Net.WebException' occurred in System.Windows.dll

WebException的堆栈跟踪如下：

at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
at System.Net.Browser.ClientHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
at AudioPlaybackAgent.AudioPlayer.TrackCallback(IAsyncResult result)
at System.Net.Browser.ClientHttpWebRequest.<>c__DisplayClassa.<InvokeGetResponseCallback>b__8(Object state2)
at System.Threading.ThreadPool.WorkItem.WaitCallback_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadPool.WorkItem.doWork(Object o)
at System.Threading.Timer.ring()

进一步深入了解trackRequest对象，我发现：

ResponseStatusCode = 'trackRequest.ResponseStatusCode' threw an exception of type 'System.NullReferenceException'

进一步说，我发现了这个：

at System.Net.HttpWebRequest.get_ResponseStatusCode()
at AudioPlaybackAgent.AudioPlayer.TrackCallback(IAsyncResult result)
at System.Net.Browser.ClientHttpWebRequest.<>c__DisplayClassa.<InvokeGetResponseCallback>b__8(Object state2)
at System.Threading.ThreadPool.WorkItem.WaitCallback_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadPool.WorkItem.doWork(Object o)
at System.Threading.Timer.ring()

这是我正在使用的代码。 TrackTimerTick函数由Timer定时器每20秒调用一次。

public static void TrackTimerTick(object state) {
        try {
            if (BackgroundAudioPlayer.Instance.PlayerState == PlayState.Playing) {
                // Create a HttpWebrequest object to the desired URL.
                HttpWebRequest trackRequest = (HttpWebRequest)HttpWebRequest.Create("<track/artist url");
                // Start the asynchronous request.
                IAsyncResult result = (IAsyncResult)trackRequest.BeginGetResponse(new AsyncCallback(TrackCallback), trackRequest);
            }
        } catch (WebException e) {
            Debug.WriteLine(e.Message);
        } catch (Exception e) {
            Debug.WriteLine(e.Message);
        }
    }


    public static void TrackCallback(IAsyncResult result) {
        // State of request is asynchronous.
        HttpWebRequest trackRequest = (HttpWebRequest)result.AsyncState;
        HttpWebResponse trackResponse = (HttpWebResponse)trackRequest.EndGetResponse(result); // WebException thrown here

        using (StreamReader httpwebStreamReader = new StreamReader(trackResponse.GetResponseStream())) {
            string results = httpwebStreamReader.ReadToEnd();
            XDocument trackXml = XDocument.Load(results);

            string title = (from t in trackXml.Descendants("channel") select t.Element("title").Value).First<string>();
            string artist = (from t in trackXml.Descendants("channel") select t.Element("artist").Value).First<string>();
            if (BackgroundAudioPlayer.Instance.Track != null) {
                AudioTrack track = BackgroundAudioPlayer.Instance.Track;
                track.BeginEdit();
                track.Title = title;
                track.Artist = artist;
                track.EndEdit();
            }

        }
        trackResponse.Close();


    }

任何人都可以帮我解决这个问题吗？提前谢谢。

Answer 1

问题是您在响应到达之前调用NotifyComplete（）。我不完全了解会发生什么，但你发起请求，你调用NotifyComplete，操作系统冻结了代理的进程，然后下一次代理唤醒时，WebClient立即抛出异常，可能是设计。

因此解决方案是在得到响应之前不要调用NotifyComplete。

HttpWebRequest返回null ResponseStatusCode

1 个答案: