使用 HttpWebRequest 时，为什么我在某些链接上收到“(304) Not Modified”错误？

任何想法为什么在我尝试使用 HttpWebRequest 访问的某些链接上我收到“远程服务器返回错误：(304) 未修改”。在代码中？

我使用的代码来自杰夫的帖子在这里（该页面似乎消失了，请参阅在 Wayback Machine 上存档副本).

请注意，代码的概念是一个简单的代理服务器，因此我将浏览器指向这段本地运行的代码，该代码获取浏览器的请求，然后通过创建一个新的 HttpWebRequest 来代理它，如您将在代码。它对于大多数网站/链接都适用，但对于某些网站/链接来说，会出现此错误。您将看到代码中的一个关键位，它似乎将 http 标头设置从浏览器请求复制到向站点发出的请求，并复制标头属性。不确定问题是否与它如何模仿请求的这方面有关，然后结果返回后会发生什么？

case "If-Modified-Since":
   request.IfModifiedSince = DateTime.Parse(listenerContext.Request.Headers[key]);
   break;

例如，我从以下位置得到这个问题http://en.wikipedia.org/wiki/Main_Page

附言。在这里更新

还是解决不了这个问题基本上我可以识别出 1 个有问题的链接，并且似乎工作正常，第二次出现错误，第三次正常，第四次出现错误，第五次正常等等。好像有一些状态没有被清除或代码中的一些东西。我尝试使用“using”类型语句等稍微清理一下代码。

这是代码。如果有人能发现为什么我每第二次浏览到这样的链接http://newsimg.bbc.co.uk/css/screen/1_0_16/nol/v4/story.css（从第二次开始，不是第一次）通过这个代理代码，我得到了我很想听到的错误。

class Program
{
    static void Main(string[] args)
    {
        Proxy p = new Proxy(8080);

        Thread proxythread = new Thread(new ThreadStart(p.Start));
        proxythread.Start();

        Console.WriteLine("Proxy Started. Press Any Key To Stop...");
        Console.ReadKey();

        p.Stop();
     }
}

public class Proxy
{
    private HttpListener _listener;
    private int _port;

    public Proxy(int port)
    {
        int defaultport = 8080;

        // Setup Thread Pool
        System.Threading.ThreadPool.SetMaxThreads(50, 1000);
        System.Threading.ThreadPool.SetMinThreads(50, 50);

        // Sanitize Port Number
        if (port < 1024 || port > 65535)
            port = defaultport;

        // Create HttpListener Prefix
        string prefix = string.Format("http://*:{0}/", port);
        _port = port;

        // Create HttpListener
        _listener = new HttpListener();
        _listener.Prefixes.Add(prefix);
    }

    public void Start()
    {
        _listener.Start();

        while (true)
        {
            HttpListenerContext request = null;

            try
            {
                request = _listener.GetContext();

                // Statistics (by Greg)
                int availThreads = -1;
                int compPortThreads = -1;
                ThreadPool.GetAvailableThreads(out availThreads, out compPortThreads);
                log("INFO", request.Request.Url.ToString(), "START - [" + availThreads + "]");

                ThreadPool.QueueUserWorkItem(ProcessRequest, request);
            }
            catch (HttpListenerException ex)
            {
                log("ERROR", "NA", "INFO: HttpListenerException - " + ex.Message);
                break;
            }
            catch (InvalidOperationException ex)
            {
                log("ERROR", "NA", "INFO: InvalidOperationException - " + ex.Message);
                break;
            }
        }
    }

    public void Stop()
    {
        _listener.Stop();
    }

    private void log(string sev, string uri, string message)
    {
        Console.Out.WriteLine(Process.GetCurrentProcess().Id + " - " + sev + " (" + uri + "): " + message);
    }

    private void ProcessRequest(object _listenerContext)
    {
        #region local variables
        HttpWebRequest psRequest;                   // Request to send to remote web server
        HttpWebResponse psResponse;                 // Response from remote web server         
        List<byte> requestBody = new List<byte>();  // Byte array to hold the request's body
        List<byte> responseBody = new List<byte>(); // Byte array to hold the response's body
        byte[] buffer;
        string uri = "";
        #endregion

        var listenerContext = (HttpListenerContext)_listenerContext;
        uri = listenerContext.Request.Url.ToString().Replace(string.Format(":{0}", _port), "");

        // Create Interent Request 
        HttpWebRequest internetRequest = (HttpWebRequest)WebRequest.Create(uri);
        #region Build Request Up
        internetRequest.Method = listenerContext.Request.HttpMethod;
        internetRequest.ProtocolVersion = listenerContext.Request.ProtocolVersion;
        internetRequest.UserAgent = listenerContext.Request.UserAgent;
        foreach (string key in listenerContext.Request.Headers.AllKeys)
        {
            try
            {
                switch (key)
                {
                    case "Proxy-Connection":
                    case "Connection":
                        internetRequest.KeepAlive = (listenerContext.Request.Headers[key].ToLower() == "keep-alive") ? true : false;
                        break;

                    case "Content-Length":
                        internetRequest.ContentLength = listenerContext.Request.ContentLength64;
                        break;

                    case "Content-Type":
                        internetRequest.ContentType = listenerContext.Request.ContentType;
                        break;

                    case "Accept":
                        internetRequest.Accept = listenerContext.Request.Headers[key];
                        break;

                    case "Host":
                        break;

                    case "Referer":
                        internetRequest.Referer = listenerContext.Request.Headers[key];
                        break;

                    case "If-Modified-Since":
                        internetRequest.IfModifiedSince = DateTime.Parse(listenerContext.Request.Headers[key]);
                        break;

                    default:
                        internetRequest.Headers.Add(key, listenerContext.Request.Headers[key]);
                        break;
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error settup up psRequest object. Error = " + ex.Message + "\n" + ex.StackTrace);
            }
        }
        #endregion

        #region Copy content into request
        buffer = new byte[1024];
        using (Stream instream = listenerContext.Request.InputStream)
        {
            int incount = instream.Read(buffer, 0, buffer.Length);
            while (incount > 0)
            {
                internetRequest.GetRequestStream().Write(buffer, 0, incount);
                incount = instream.Read(buffer, 0, buffer.Length);
            }
        }
        #endregion

        // Get Internet Response
        HttpWebResponse internetResponse = null;
        try
        {
            using (internetResponse = (HttpWebResponse)internetRequest.GetResponse())
            {
                #region Configure Local Response Header Keys
                foreach (string key in internetResponse.Headers.Keys)
                {
                    try
                    {
                        switch (key)
                        {
                            case "Transfer-Encoding":
                                listenerContext.Response.SendChunked = (internetResponse.Headers[key].ToLower() == "chunked") ? true : false;
                                break;

                            case "Content-Length":
                                listenerContext.Response.ContentLength64 = internetResponse.ContentLength;
                                break;

                            case "Content-Type":
                                listenerContext.Response.ContentType = internetResponse.Headers[key];
                                break;

                            case "Keep-Alive":
                                listenerContext.Response.KeepAlive = true;
                                break;

                            default:
                                listenerContext.Response.Headers.Add(key, internetResponse.Headers[key]);
                                break;
                        }
                    }
                    catch (Exception ex)
                    {
                        log("ERROR", uri, "Error settup up listenerContext.Response objects. Error = " + ex.Message + "\n" + ex.StackTrace);
                    }
                }
                #endregion

                try
                {
                    // Transfer the body data from Internet Response to Internal Response
                    buffer = new byte[1024];
                    using (Stream inputStream = internetResponse.GetResponseStream())
                    {
                        int outcount = inputStream.Read(buffer, 0, buffer.Length);
                        while (outcount > 0)
                        {
                            listenerContext.Response.OutputStream.Write(buffer, 0, outcount);
                            outcount = inputStream.Read(buffer, 0, buffer.Length);
                        }
                    }
                }
                catch (Exception ex)
                {
                    log("ERROR", uri, "Could not obtain response from URI: " + ex.Message);
                }
                finally
                {
                    listenerContext.Response.OutputStream.Close();
                }
            }
        }
        catch (Exception ex)
        {
            //if (ex is InvalidOperationException ||
            //    ex is ProtocolViolationException ||
            //    ex is WebException)
            //{
            //    log(uri, "Could not successfully get response: " + ex.GetType() + " - " + ex.Message);
            //    listenerContext.Response.Close();
            //    return;
            //}
            //else { throw; }

            log("ERROR", uri, "Could not successfully get response: " + ex.GetType() + " - " + ex.Message);
            listenerContext.Response.Close();
        }
    }
}

这是我看到的一个例子 - 第一个命中很好，第二个有错误......

Proxy Started. Press Any Key To Stop...
2080 - INFO (http://newsimg.bbc.co.uk:8080/css/screen/1_0_16/nol/v4/story.css): START - [50]
2080 - INFO (http://newsimg.bbc.co.uk:8080/css/screen/1_0_16/nol/v4/story.css): START - [50]
2080 - ERROR (http://newsimg.bbc.co.uk/css/screen/1_0_16/nol/v4/story.css): Could not successfully get response: System.Net.WebException - The remote server returned an error: (304) Not Modified.

首先，这不是一个错误。这3xx表示重定向。真正的错误是4xx（客户端错误）和5xx（服务器错误）。

如果客户得到304 Not Modified，那么客户端有责任从自己的缓存中显示有问题的资源。一般来说，代理不应该担心这一点。这只是使者。