需求分析：本人最近做一个项目，项目中需要从新闻的索引页（就是上面有很多链接的那种网页），获取新闻正文页源码，并将新闻正文页源码保存到本地数据库中。
    但是由于网络稳定性的原因，总会出现 404 page not found 类型的error。（但是网页是确确实实存在的）。而且这种错误，往往是在程序运行一段时间后出现的，觉得很不可思议。我在网络上查这种问题的解决方案时，发现没有一种管用的。本人现在已经成功解决该问题，遂将自己的解决方案写下来和大家分享与探讨。
解决方案核心：一旦出现这种错误，程序中就递归调用下载函数本身。代码说明如下：

 public static string GetDataFromUrl(string url, int nRetryTimes)
        {
            if (nRetryTimes == 0)
                return string.Empty;

            string result = string.Empty;
            try
            {
                result=GetDataFromUrl(url);
            }
            catch (System.Exception exc)
            {
                if(exc.Message.IndexOf("404")!=-1)
                {
                    result=GetDataFromUrl(url,nRetryTimes-1);
                }
            }
            return result;
        }
其中nRetryTimes 代表出现这种错误后，函数递归调用自己的次数，也可以理解为递归终止的条件。GetDataFromUrl(string url)函数代码如下：

 public static string GetDataFromUrl(string url)
       {
           string str = string.Empty;
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);
            //设置http头
            request.AllowAutoRedirect = true;
            request.AllowWriteStreamBuffering = true;
            request.Referer = "";
            request.Timeout = 10000000;
            request.UserAgent = "";
            ;
            request.KeepAlive = false;//to avoid the error of time out
            HttpWebResponse response = null;
            response = (HttpWebResponse)request.GetResponse();

            //根据http应答的http头来判断编码
            string characterSet = response.CharacterSet;
            Encoding encode;
            if (characterSet != "")
            {
                if (characterSet == "ISO-8859-1")
                {
                    characterSet = "gb2312";
                }
                encode = Encoding.GetEncoding(characterSet);
            }
            else
            {
                encode = Encoding.Default;
            }

            //声明一个内存流来保存http应答流
            Stream receiveStream = response.GetResponseStream();
            MemoryStream mStream = new MemoryStream();

            byte[] bf = new byte[255];
            int count = receiveStream.Read(bf, 0, 255);
            while (count > 0)
            {
                mStream.Write(bf, 0, count);
                count = receiveStream.Read(bf, 0, 255);
            }
            receiveStream.Close();

            mStream.Seek(0, SeekOrigin.Begin);

            //从内存流里读取字符串
            StreamReader reader = new StreamReader(mStream, encode);
            char[] buffer = new char[1024];
            count = reader.Read(buffer, 0, 1024);
            while (count > 0)
            {
                str += new String(buffer, 0, count);
                count = reader.Read(buffer, 0, 1024);
            }

            //从解析出的字符串里判断charset，如果和http应答的编码不一直
            //那么以页面声明的为准，再次从内存流里重新读取文本
            Regex reg =
               new Regex(@"<meta[\s\S]+?charset=(.*?)""[\s\S]+?>",
                          RegexOptions.Multiline | RegexOptions.IgnoreCase);
            MatchCollection mc = reg.Matches(str);
            if (mc.Count > 0)
            {
                string tempCharSet = mc[0].Result("$1");
                if (string.Compare(tempCharSet, characterSet, true) != 0)
                {
                    encode = Encoding.GetEncoding(tempCharSet);
                    str = string.Empty;
                    mStream.Seek(0, SeekOrigin.Begin);
                    reader = new StreamReader(mStream, encode);
                    buffer = new char[255];
                    count = reader.Read(buffer, 0, 255);
                    while (count > 0)
                    {
                        str += new String(buffer, 0, count);
                        count = reader.Read(buffer, 0, 255);
                    }
                }
            }
            reader.Close();
            mStream.Close();
            if (response != null)
                response.Close();

            return str;

       
 
         
       }

值得说明的是：尽管采用了此方法，当你查看数据库的时候，你还是会发现有些正文源码没有下载下来。拿我的数据表单来说：我的数据库表单的各个属性如下 ArticlePageId,--数据表的主键。ArticlePageTitle--新闻标题，ArticlePageUrl,--新闻正文页URL，ArticlePageSource--新闻正文页源码，也就是从ArticlePageUrl下载的源码。如果ArticlePageSource字段为空，则表明，下载失败。于是，我又加了一个打补丁的模块。代码如下：

把补丁的模块代码
 public static void PatchingUp(string desTable)
        {
            string conStr = "server=(local);database=ArticleCollection;uid=sa;pwd=123456";
            string des = desTable;
            string select = string.Format("SELECT ArticlePageId,ArticlePageSource,ArticlePageUrl from {0}", des);
            SqlConnection connection = new SqlConnection(conStr);
            SqlDataAdapter adapter = new SqlDataAdapter(select, connection);
            SqlCommandBuilder builder = new SqlCommandBuilder(adapter);
            DataTable table = new DataTable();
            adapter.Fill(table);
            bool isModified = false;
            for (int i = 0; i < table.Rows.Count; i++)
            {
                if (table.Rows[i]["ArticlePageSource"].ToString() == null || table.Rows[i]["ArticlePageSource"].ToString() == "")
                {
                    Console.WriteLine("在原数据库第{0}行插入失败", i+1);
                    string url = table.Rows[i]["ArticlePageUrl"].ToString().Trim();
                    table.Rows[i]["ArticlePageSource"] = ClawlerAssist.GetDataFromUrl(url, 5);

                    Console.WriteLine("在原数据库第{0}行更新成功", i+1);
                    if (!isModified)
                    {
                        isModified = true;
                    }

                }
            }
            if (isModified)
            {
                adapter.Update(table);
            }    
 
        }