使用 HttpWebRequest 和 StreamReader 下载网页时出现未处理的异常

Question

这是一个使用 .NET 4.0 的 C# 应用程序。

背景：

我一直在尝试改进从多个 Web 服务器下载文件的应用程序的一部分。除了远程计算机关闭连接的偶尔情况外，一切都运行良好。我认为这可能是由于多种因素造成的，包括网络问题、电源问题等。如果连接关闭，我想简单地跳过下载。然而，目前应用程序会导致一个异常，该异常由AppDomain.CurrentDomain.UnhandledException 的处理程序拾取。堆栈跟踪如下：

堆栈跟踪：

Unable to read data from the transport connection:
An existing connection was forcibly closed by the remote host.
at System.Net.ConnectStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at System.IO.StreamReader.ReadBuffer() at System.IO.StreamReader.ReadToEnd()
at ProjectName.ClassNetwork.DownloadFile(String _IP, Int32 _Port, String _File)
at ProjectName.FormMain.GetIFile(ClassSensor Sensor, String& RawHolder, String[] FileData)
at ProjectName.FormMain.GetLegacyData(ClassSensor Sensor)
at ProjectName.FormMain.threader_Download(Object SensorObject)
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart(Object obj)

我最初有 WebException 和 Exception 的 try/catch，但他们没有捕捉到这个特定的错误。我添加了最里面的 try/catch 试图避免这个问题，但无济于事。我应该提到这个方法是由 UI 线程以外的线程调用的，这可能是 try/catch 块似乎不起作用的部分原因。在多线程错误捕获方面，请原谅我是新手！

问题：

• 如何改进此方法并正确处理中途连接失败的情况？

---

代码：

private static object[] DownloadFile(string _IP, int _Port, string _File)
{
    string uri = String.Format("http://{0}:{1}/{2}", _IP, _Port, _File);
    string Status = String.Empty;
    string Data = String.Empty;
    HttpWebResponse Response = null;

    try
    {
        HttpWebRequest webReq = (HttpWebRequest) WebRequest.Create(uri);
        webReq.Timeout = 10000;
        webReq.ReadWriteTimeout = 30000;
        Response = (HttpWebResponse) webReq.GetResponse();
        using (Stream dataStream = Response.GetResponseStream())
            if (dataStream != null)
                using (StreamReader reader = new StreamReader(dataStream))
                    try
                    {
                        // This line causes crashes if remote computer closes connection
                        Data = reader.ReadToEnd();
                    }
                    catch { } // Inner try/catch added to no avail
        Response.Close();
    }
    catch (WebException exc)
    {
        // Connection Error
        Status = exc.Status.ToString();
        Data = String.Empty;
    }
    catch (Exception exc)
    {
        // Other error
        Status = exc.Message;
        Data = String.Empty;
    }

    if (Response != null && Response.StatusCode != HttpStatusCode.OK)
    {
        Status = Response.StatusCode.ToString();
        Data = String.Empty;
    }

    return new object[] { Status, Data };
}

跟踪日志：

根据 Feroze 的建议，我运行了一个跟踪点，结果如下：

System.Net Error: 0 : [2164] Exception in the
#46104728::UnhandledExceptionHandler - Stream was not readable.
System.Net Error: 0 : [2164]     at System.IO.BinaryReader..ctor(Stream input, Encoding encoding)
at ProjectName.ClassNetwork.DLStream(ClassSensor Sensor)
at ProjectName.ClassNetwork.DownloadFile(ClassSensor Sensor)
at ProjectName.FormMain.GetStreamFile(ClassSensor Sensor)
at ProjectName.FormMain.threader_Download(Object SensorObject)
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncctx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart(Object obj)

Answer 1

从堆栈中，我看不出这是您无法捕获的线程池线程抛出的异常。显然，从堆栈看来，它似乎是由在您的应用程序线程上调用的 ConnectStream::Read 方法抛出的。