Code
using System;

public class basePage:System.Web.UI.Page 
{
    /// 母页，作基本的初始化工作

    public basePage()
    {
        Page.Title = "我的网站";
    }


    /// 获取制定cookie的值

    public string GetCookieValue(string CookieName, string ErrorReturn)
    {
        if (Request.Cookies[CookieName] != null && !
            string.IsNullOrEmpty(Request.Cookies[CookieName].Value))
        {
            return Request.Cookies[CookieName].Value;
        }
        else
        {
            return ErrorReturn;
        }
    }

    /// 检查指定的cookie是否存在

    public bool IsCookieAvailable(string CookieName)
    {
        if (Request.Cookies[CookieName] != null && !
         string.IsNullOrEmpty(Request.Cookies[CookieName].Value))
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    ///  用户登出

    public void UserLogout()
    {
        //退出清除 Cookie
        Response.Cookies["UserName"].Value = "";
        Response.Cookies["UserID"].Value = "";

        Response.Redirect("default.aspx");
    }
}

Code
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default: BasePage 
{
    protected void Page_Load(object sender, EventArgs e)
    {
          UserLogout();
    }
}