用asp.net c# HttpWebRequest获取网页源代码

该方法需要传递目标网页的编码方式，比如System.Text.Encoding.Default或者System.Text.Encoding.UTF8

如果哪位高手知道如何自动判断目标页面的编码格式，请在评论中告知。谢谢！

/**//// <summary>
        /// 获取源代码
        /// </summary>
        /// <param name="url"></param>
        /// <param name="encoding"></param>
        /// <returns></returns>
        public static string GetPage(string url, Encoding encoding)
        {
            HttpWebRequest request = null;
            HttpWebResponse response = null;
            StreamReader reader = null;
            try
            {
                request = (HttpWebRequest)WebRequest.Create(url);
                request.UserAgent = "www.svnhost.cn";
                request.Timeout = 20000;
                request.AllowAutoRedirect = false;

                response = (HttpWebResponse)request.GetResponse();
                if (response.StatusCode == HttpStatusCode.OK && response.ContentLength < 1024 * 1024)
                {
                    reader = new StreamReader(response.GetResponseStream(), encoding);
                    string html = reader.ReadToEnd();

                    return html;
                }
            }
            catch
            {
            }
            finally
            {

                if (response != null)
                {
                    response.Close();
                    response = null;
                }
                if (reader != null)
                    reader.Close();

                if (request != null)
                    request = null;

            }

            return string.Empty;
        }