public interface ISingleLogin
    {
        string SigleUserLoginId { get; }

        void SigleUserLogout();

    }

public class BasePage : System.Web.UI.Page , BNet.Web.Modulers.ISingleLogin
{
    public BasePage()
    {
        //
        // TODO: 在此处添加构造函数逻辑
        //
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        if (Session["UserId"] == null)
        {
            Response.Write("你还没有登陆");
            Response.Redirect("login.aspx");
        }
    }

    ISingleLogin 成员#region ISingleLogin 成员

    public string SigleUserLoginId
    {
        get 
        {
            if (Session["UserId"] != null)
            {
                return Session["UserId"].ToString();
            }
            else
                return "";
        }
    }

    public void SigleUserLogout()
    {
        Session.Abandon();
        Response.Write("你在别处已经登陆，强制退出本次登陆!");
    }

    #endregion
}

<system.web>
    <httpModules>
      <add name="SingleLogin" type="BNet.Web.Modulers.SingleLoginModuler"/>

    </httpModules>
</system.web>

using System;
using System.Collections.Generic;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.UI;

namespace BNet.Web.Modulers
{
    /**//// <summary>
    /// SingleLoginModuler 的摘要说明
    /// </summary>
    public class SingleLoginModuler : System.Web.IHttpModule
    {
        const string sigle_login_userid = "evlon_siglelogin_userid";
        const string sigle_pre_logout_sessionid = "evlon_sigle_pre_logout_sessionid";

        static StringLifeValueDictionary loginedUserIdDictionary = null;
        static StringLifeValueDictionary LoginedUserIdDictionary
        {
            get
            {
                if (loginedUserIdDictionary == null)
                {
                    loginedUserIdDictionary = new StringLifeValueDictionary();
                }
                else
                {
                    List<string> listRemove = new List<string>();
                    StringLifeValueDictionary.Enumerator iter = loginedUserIdDictionary.GetEnumerator();
                    while (iter.MoveNext())
                    {
                        if (iter.Current.Value.life < DateTime.Now)
                        {
                            listRemove.Add(iter.Current.Key);
                        }
                    }

                    foreach (string key in listRemove)
                    {
                        loginedUserIdDictionary.Remove(key);
                    }
                }

                return loginedUserIdDictionary;
            }
        }

        static StringLifeValueDictionary preLogoutSessionIdDictionary = null;
        static StringLifeValueDictionary PreLogoutSessionIdDictionary
        {
            get
            {
                if (preLogoutSessionIdDictionary == null)
                {
                    preLogoutSessionIdDictionary = new StringLifeValueDictionary();
                }
                else
                {
                    List<string> listRemove = new List<string>();
                    StringLifeValueDictionary.Enumerator iter = preLogoutSessionIdDictionary.GetEnumerator();
                    while (iter.MoveNext())
                    {
                        if (iter.Current.Value.life < DateTime.Now)
                        {
                            listRemove.Add(iter.Current.Key);
                        }
                    }

                    foreach (string key in listRemove)
                    {
                        preLogoutSessionIdDictionary.Remove(key);
                    }
                }

                return preLogoutSessionIdDictionary;
            }
        }

        public SingleLoginModuler()
        {
            //
            // TODO: 在此处添加构造函数逻辑
            //
        }

        IHttpModule 成员#region IHttpModule 成员

        public void Dispose()
        {
        }

        public void Init(HttpApplication context)
        {
            context.PreRequestHandlerExecute += new EventHandler(context_PreRequestHandlerExecute);
            context.PostRequestHandlerExecute += new EventHandler(context_PostRequestHandlerExecute);
            
        }



        void context_PreRequestHandlerExecute(object sender, EventArgs e)
        {
            HttpApplication context = sender as HttpApplication;
            IHttpHandler httpHandler = context.Context.CurrentHandler;
            if (httpHandler is ISingleLogin)
            {
                ISingleLogin sl = httpHandler as ISingleLogin;
                string suid = sl.SigleUserLoginId;
                if (suid != string.Empty)
                {
                    if (PreLogoutSessionIdDictionary.ContainsKey(context.Session.SessionID))
                    {
                        //这个用户应该强制登出
                        PreLogoutSessionIdDictionary.Remove(context.Session.SessionID);

                        Page page = (Page)httpHandler;
                        page.PreInit += new EventHandler(page_PreInit);

                    }
                    else if (!LoginedUserIdDictionary.ContainsKey(suid))
                    {
                        LoginedUserIdDictionary.Add(suid, new LifeValue(context.Session.SessionID));
                    }
                }
            }

        }

        void page_PreInit(object sender, EventArgs e)
        {
            Page page = sender as Page;
            ISingleLogin sl = page as ISingleLogin;
            if (sl != null)
            {
                sl.SigleUserLogout();
                page.Response.End();
            }
        }

        void context_PostRequestHandlerExecute(object sender, EventArgs e)
        {
            //从LogineduserId 里找到和当前用户一样的用户ID的SessionId
            HttpApplication context = sender as HttpApplication;
            IHttpHandler httpHandler = context.Context.CurrentHandler;
            if (httpHandler is ISingleLogin)
            {
                ISingleLogin sl = httpHandler as ISingleLogin;
                string suid = sl.SigleUserLoginId;
                if (suid != string.Empty)
                {
                    if (LoginedUserIdDictionary.ContainsKey(suid))
                    {
                        string sessionId = LoginedUserIdDictionary[suid].value;
                        if (sessionId != context.Session.SessionID)
                        {
                            if (!PreLogoutSessionIdDictionary.ContainsKey(sessionId))
                            {
                                PreLogoutSessionIdDictionary.Add(sessionId,new LifeValue(suid));
                            }

                            LoginedUserIdDictionary.Remove(suid);
                        }
                    }
                    else
                    {
                        LoginedUserIdDictionary.Add(sl.SigleUserLoginId, new LifeValue( context.Session.SessionID));
                    }

                }
            }
        }

        #endregion
    }

    public class LifeValue
    {
        public string value;
        public DateTime life;

        public LifeValue(string value)
        {
            this.value = value;
            this.life = DateTime.Now.AddMinutes(HttpContext.Current.Session.Timeout + 5);
        }
    }

    public class StringLifeValueDictionary : Dictionary<string, LifeValue>
    {

    }


    public interface ISingleLogin
    {
        string SigleUserLoginId { get; }

        void SigleUserLogout();

    }
}