如何使用 Html Agility Pack 使请求超时

Question

我正在向当前离线（故意）的远程 Web 服务器发出请求。

我想找出使请求超时的最佳方法。基本上，如果请求运行时间超过“X”毫秒，则退出请求并返回 null 响应。

目前，网络请求只是坐在那里等待响应......

我将如何最好地解决这个问题？

这是当前的代码片段

    public JsonpResult About(string HomePageUrl)
    {
        Models.Pocos.About about = null;
        if (HomePageUrl.RemoteFileExists())
        {
            // Using the Html Agility Pack, we want to extract only the
            // appropriate data from the remote page.
            HtmlWeb hw = new HtmlWeb();
            HtmlDocument doc = hw.Load(HomePageUrl);
            HtmlNode node = doc.DocumentNode.SelectSingleNode("//div[@class='wrapper1-border']");

            if (node != null)
            { 
                about = new Models.Pocos.About { html = node.InnerHtml };
            }
                //todo: look into whether this else statement is necessary
            else 
            {
                about = null;
            }
        }

        return this.Jsonp(about);
    }

Answer 1

通过以下方法检索您的 url 网页：

private static string retrieveData(string url)
    {
        // used to build entire input
        StringBuilder sb = new StringBuilder();

        // used on each read operation
        byte[] buf = new byte[8192];

        // prepare the web page we will be asking for
        HttpWebRequest request = (HttpWebRequest)
        WebRequest.Create(url);
        request.Timeout = 10; //10 millisecond
        // execute the request

        HttpWebResponse response = (HttpWebResponse)
        request.GetResponse();

        // we will read data via the response stream
        Stream resStream = response.GetResponseStream();

        string tempString = null;
        int count = 0;

        do
        {
            // fill the buffer with data
            count = resStream.Read(buf, 0, buf.Length);

            // make sure we read some data
            if (count != 0)
            {
                // translate from bytes to ASCII text
                tempString = Encoding.ASCII.GetString(buf, 0, count);

                // continue building the string
                sb.Append(tempString);
            }
        }
        while (count > 0); // any more data to read?

        return sb.ToString();
    }

并使用 HTML Agility 包并检索 html 标签，如下所示：

public static string htmlRetrieveInfo()
    {
        string htmlSource = retrieveData("http://example.com/test.html");
        HtmlDocument doc = new HtmlDocument();
        doc.LoadHtml(htmlSource);
        if (doc.DocumentNode.SelectSingleNode("//body") != null)
        {
          HtmlNode node = doc.DocumentNode.SelectSingleNode("//body");
        }
        return node.InnerHtml;
    }

Answer 2

Html Agility Pack 是开源的。这就是为什么你可以自己修改源代码。
首先将此代码添加到类 HtmlWeb 中：

private int _timeout = 20000;

public int Timeout 
    { 
        get { return _timeout; } 
        set
        {
            if (_timeout < 1) 
                throw new ArgumentException("Timeout must be greater then zero.");
            _timeout = value;
        }
    }

然后找到此方法

private HttpStatusCode Get(Uri uri, string method, string path, HtmlDocument doc, IWebProxy proxy, ICredentials creds)

并修改它：

req = WebRequest.Create(uri) as HttpWebRequest;
req.Method = method;
req.UserAgent = UserAgent;
req.Timeout = Timeout; //add this

或者类似的东西：

htmlWeb.PreRequest = request =>
            {
                request.Timeout = 15000;
                return true;
            };

Answer 3

我必须对最初发布的代码进行一些小调整

    public JsonpResult About(string HomePageUrl)
    {
        Models.Pocos.About about = null;
        // ************* CHANGE HERE - added "timeout in milliseconds" to RemoteFileExists extension method.
        if (HomePageUrl.RemoteFileExists(1000))
        {
            // Using the Html Agility Pack, we want to extract only the
            // appropriate data from the remote page.
            HtmlWeb hw = new HtmlWeb();
            HtmlDocument doc = hw.Load(HomePageUrl);
            HtmlNode node = doc.DocumentNode.SelectSingleNode("//div[@class='wrapper1-border']");

            if (node != null)
            { 
                about = new Models.Pocos.About { html = node.InnerHtml };
            }
                //todo: look into whether this else statement is necessary
            else 
            {
                about = null;
            }
        }

        return this.Jsonp(about);
    }

然后我修改了我的 RemoteFileExists 扩展方法以设置超时

    public static bool RemoteFileExists(this string url, int timeout)
    {
        try
        {
            //Creating the HttpWebRequest
            HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;

            // ************ ADDED HERE
            // timeout the request after x milliseconds
            request.Timeout = timeout;
            // ************

            //Setting the Request method HEAD, you can also use GET too.
            request.Method = "HEAD";
            //Getting the Web Response.
            HttpWebResponse response = request.GetResponse() as HttpWebResponse;
            //Returns TRUE if the Status code == 200
            return (response.StatusCode == HttpStatusCode.OK);
        }
        catch
        {
            //Any exception will returns false.
            return false;
        }
    }

在这种方法中，如果我的超时在 RemoteFileExists 可以确定之前触发标头响应，那么我的 bool 将返回 false。

Answer 4

您可以使用标准 HttpWebRequest 来获取远程资源并设置 超时属性。如果生成的 HTML 成功，则将其提供给 HTML Agility Pack 进行解析。