两段自动检测网页编码的C#代码

使用C#访问网页时，如何检测网页的编码是必须解决的问题。实测下面两段代码并不是百分百可靠，某些页面仍可能识别错误。在HttpWebResponse对象的几个编码属性都不准确时，网页meta中的charset值可用，然而在无法识别页面代码的情况下，如何正确解析出来meta标签的charset值呢？ 不知浏览器内部是如何自动识别的，以后会关注。

代码1：

public static String DecodeData(WebResponse w)
        {
            //
            // first see if content length header has charset = calue
            //
            String charset = null;
            String ctype = w.Headers["content-type"];
            if (ctype != null)
            {
                int ind = ctype.IndexOf("charset=");
                if (ind != -1)
                {
                    charset = ctype.Substring(ind + 8);
                    Console.WriteLine("CT: charset=" + charset);
                }
            }
            // save data to a memorystream
            MemoryStream rawdata = new MemoryStream();
            byte[] buffer = new byte[1024];
            Stream rs = w.GetResponseStream();
            int read = rs.Read(buffer, 0, buffer.Length);
            while (read > 0)
            {
                rawdata.Write(buffer, 0, read);
                read = rs.Read(buffer, 0, buffer.Length);
            }
            rs.Close();
            //
            // if ContentType is null, or did not contain charset, we search in body
            //
            if (charset == null)
            {
                MemoryStream ms = rawdata;
                ms.Seek(0, SeekOrigin.Begin);
                StreamReader srr = new StreamReader(ms, Encoding.ASCII);
                String meta = srr.ReadToEnd();
                if (meta != null)
                {
                    int start_ind = meta.IndexOf("charset=");
                    int end_ind = -1;
                    if (start_ind != -1)
                    {
                        end_ind = meta.IndexOf("\"", start_ind);
                        if (end_ind != -1)
                        {
                            int start = start_ind + 8;
                            charset = meta.Substring(start, end_ind - start + 1);
                            charset = charset.TrimEnd(new Char[] { '>', '"' });
                            Console.WriteLine("META: charset=" + charset);
                        }
                    }
                }
            }
            Encoding e = null;
            if (charset == null)
            {
                e = Encoding.ASCII; //default encoding
            }
            else
            {
                try
                {
                    e = Encoding.GetEncoding(charset);
                }
                catch (Exception ee)
                {
                    Console.WriteLine("Exception: GetEncoding: " + charset);
                    Console.WriteLine(ee.ToString());
                    e = Encoding.ASCII;
                }
            }
            rawdata.Seek(0, SeekOrigin.Begin);
            StreamReader sr = new StreamReader(rawdata, e);
            String s = sr.ReadToEnd();
            return s.ToLower();
        }

代码2：

public static  string DownloadPage(string url)
        {
            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
            using (HttpWebResponse resp = (HttpWebResponse)req.GetResponse())
            {
                byte[] buffer;
                using (Stream s = resp.GetResponseStream())
                {
                    buffer = ReadStream(s);
                }
                string pageEncoding = "";
                Encoding e = Encoding.UTF8;
                if (resp.ContentEncoding != "")
                    pageEncoding = resp.ContentEncoding;
                else if (resp.CharacterSet != "")
                    pageEncoding = resp.CharacterSet;
                else if (resp.ContentType != "")
                    pageEncoding = GetCharacterSet(resp.ContentType);
                if (pageEncoding == "")
                    pageEncoding = GetCharacterSet(buffer);
                if (pageEncoding != "")
                {
                    try
                    {
                        e = Encoding.GetEncoding(pageEncoding);
                    }
                    catch
                    {
                        return null;
                    }
                }
                string data = e.GetString(buffer);
                return data;
            }
        }
        private static string GetCharacterSet(string s)
        {
            s = s.ToUpper();
            int start = s.LastIndexOf("CHARSET");
            if (start == -1)
                return "";
            start = s.IndexOf("=", start);
            if (start == -1)
                return "";
            start++;
            s = s.Substring(start).Trim();
            int end = s.Length;
            int i = s.IndexOf(";");
            if (i != -1)
                end = i;
            i = s.IndexOf("\"");
            if (i != -1 && i < end)
                end = i;
            i = s.IndexOf("'");
            if (i != -1 && i < end)
                end = i;
            i = s.IndexOf("/");
            if (i != -1 && i < end)
                end = i;
            return s.Substring(0, end).Trim();
        }
        private static  string GetCharacterSet(byte[] data)
        {
            string s = Encoding.Default.GetString(data);
            return GetCharacterSet(s);
        }
        private static byte[] ReadStream(Stream s)
        {
            try
            {
                byte[] buffer = new byte[8096];
                using (MemoryStream ms = new MemoryStream())
                {
                    while (true)
                    {
                        int read = s.Read(buffer, 0, buffer.Length);
                        if (read <= 0)
                        {
                            return ms.ToArray();
                        }
                        ms.Write(buffer, 0, read);
                    }
                }
            }
            catch (Exception)
            {
                return null;
            }
        }

两段代码都来自国外论坛，也许分析读取到的页面字符流本身特征可以找出匹配的编码？这就相当麻烦了。