判断QQ是否在线的实现：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net;
using System.IO;
using System.Text;

namespace qqIsonline
{
    public partial class index : System.Web.UI.Page
    {
        public string sendURL;
        protected void Page_Load(object sender, EventArgs e)
        {
            string qqnum = "2669743225";
            string result = GetQQStatue(qqnum);
            if (result == "1")
            {
                Response.Write("在线");
            }
            else
            {
                Response.Write("不在线");
            }
            
        }

        /// <summary>
        /// 获得qq是否在线的状态
        /// <param name="qqnum">要判断的q号码</param>
        /// <returns>online[0]=X的字符串，x=1或者x=0</returns>
        public string GetURL(string qqnum)
        {
            string resultStr;
            try
            {
                sendURL = "http://webpresence.qq.com/getonline?Type=1&" + qqnum + ":";
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(sendURL);  //创建一个HttpWebRequest对象
                HttpWebResponse respone = (HttpWebResponse)request.GetResponse();    //返回来自 Internet 资源的响应。
                //respone.GetResponseStream()  获取用于写入请求数据的 Stream 对象
                using (StreamReader stream = new StreamReader(respone.GetResponseStream(), Encoding.Default))
                {
                    //获取结果
                    resultStr = stream.ReadToEnd();   //读取数据
                    return resultStr;
                }
            }
            catch (Exception ex)
            {

                Response.Write(ex.Message);    //输出错误信息
            }

            return "";
        }

        /// <summary>
        /// 获得判定qq是否在线的关键字：0表示不在线，1表示在线
        /// </summary>
        /// <param name="qqnum">要判断的QQ号</param>
        /// <returns>1或是0</returns>
        public string GetQQStatue(string qqnum)
        {
            string str = GetURL(qqnum);
            int index = str.IndexOf(‘=‘);
            string strQQStute = str.Substring(index + 1, 1);
            return strQQStute;
        }
    }
}