转载：http://www.wzsky.net/html/Program/net/26849.html
using System;
using System.Xml;
using System.Text;
using System.Net;
using System.IO;
using System.Collections;
using System.Text.RegularExpressions; 

namespace test
{
    class Program
    {
        static void Main(string[] args)
        {
            string strCode;
            ArrayList alLinks;

            Console.Write("请输入一个网页地址：");
            string strURL = Console.ReadLine();
            if (strURL.Substring(0, 7) != @"http://")
            {
                strURL = @"http://" + strURL;
            }

            Console.WriteLine("正在获取页面代码，请稍侯...");
            strCode = GetPageSource(strURL);

            Console.WriteLine("正在提取超链接，请稍侯...");
            alLinks = GetHyperLinks(strCode);

            Console.WriteLine("正在写入文件，请稍侯...");
            WriteToXml(strURL, alLinks);
        }

        // 获取指定网页的HTML代码 
        static string GetPageSource(string URL)
        {
            Uri uri = new Uri(URL);

            HttpWebRequest hwReq = (HttpWebRequest)WebRequest.Create(uri);
            HttpWebResponse hwRes = (HttpWebResponse)hwReq.GetResponse();

            hwReq.Method = "Get";

            hwReq.KeepAlive = false;

            StreamReader reader = new StreamReader(hwRes.GetResponseStream(), System.Text.Encoding.GetEncoding("GB2312"));

            return reader.ReadToEnd();
        }

        // 提取HTML代码中的网址 
        static ArrayList GetHyperLinks(string htmlCode)
        {
            ArrayList al = new ArrayList();

            string strRegex = @"http://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?";

            Regex r = new Regex(strRegex, RegexOptions.IgnoreCase);
            MatchCollection m = r.Matches(htmlCode);

            for (int i = 0; i <= m.Count - 1; i++)
            {
                bool rep = false;
                string strNew = m[i].ToString();

                // 过滤重复的URL 
                foreach (string str in al)
                {
                    if (strNew == str)
                    {
                        rep = true;
                        break;
                    }
                }

                if (!rep) al.Add(strNew);
            }

            al.Sort();

            return al;
        }

        // 把网址写入xml文件 
        static void WriteToXml(string strURL, ArrayList alHyperLinks)
        {
            XmlTextWriter writer = new XmlTextWriter("HyperLinks.xml", Encoding.UTF8);

            writer.Formatting = Formatting.Indented;
            writer.WriteStartDocument(false);
            writer.WriteDocType("HyperLinks", null, "urls.dtd", null);
            writer.WriteComment("提取自" + strURL + "的超链接");
            writer.WriteStartElement("HyperLinks");
            writer.WriteStartElement("HyperLinks", null);
            writer.WriteAttributeString("DateTime", DateTime.Now.ToString());


            foreach (string str in alHyperLinks)
            {
                string title = GetDomain(str);
                string body = str;
                writer.WriteElementString(title, null, body);
            }

            writer.WriteEndElement();
            writer.WriteEndElement();

            writer.Flush();
            writer.Close();
        }

        // 获取网址的域名后缀 
        static string GetDomain(string strURL)
        {
            string retVal;

            string strRegex = @"(\.com/|\.net/|\.cn/|\.org/|\.gov/)";

            Regex r = new Regex(strRegex, RegexOptions.IgnoreCase);
            Match m = r.Match(strURL);
            retVal = m.ToString();

            strRegex = @"\.|/$";
            retVal = Regex.Replace(retVal, strRegex, "").ToString();

            if (retVal == "")
                retVal = "other";

            return retVal;
        }
    } 
}