登录页面新建一个方法名，传入用户ID，并引用此方法即可

         //单点登录
        private void isLogined(string id)
        {
            Hashtable hOnline = (Hashtable)Application["Online"];
            if (hOnline != null)
            {
                int i = 0;
                while (i < hOnline.Count)
                {
                    IDictionaryEnumerator idE = hOnline.GetEnumerator();
                    string strKey = "";
                    while (idE.MoveNext())
                    {
                        if (idE.Value != null && idE.Value.ToString().Equals(id))
                        {
                            //already login              
                            strKey = idE.Key.ToString();
                            hOnline[strKey] = "XXXXXX";
                            break;
                        }
                    }
                    i = i + 1;
                }
            }
            else
            {
                hOnline = new Hashtable();
            }
            hOnline[Session.SessionID] = id;
            Application.Lock();
            Application["Online"] = hOnline;
            Application.UnLock();
        }

新建一个CommonPage页面，系统中所有的页面都继承于CommonPage页

public partial class CommonPage : System.Web.UI.Page
	{
        public CommonPage()
        {
            //  
            // TODO: 在此处添加构造函数逻辑  
            //  
        }
        override protected void OnInit(EventArgs e)
        {
            Hashtable hOnline = (Hashtable)Application["Online"];
            if (hOnline != null)
            {
                IDictionaryEnumerator idE = hOnline.GetEnumerator();
                while (idE.MoveNext())
                {
                    if (idE.Key != null && idE.Key.ToString().Equals(Session.SessionID))
                    {
                        if (idE.Value != null && "XXXXXX".Equals(idE.Value.ToString()))
                        {
                            hOnline.Remove(Session.SessionID);
                            Application.Lock();
                            Application["Online"] = hOnline;
                            Application.UnLock();
                            string js = "<script language=javascript>alert('{0}');parent.location.href='{1}';</script>";
                            Response.Write(string.Format(js, "帐号已在别处登录 ，你将被强迫下线（若非本人登录，请注意保护密码安全）！", "login.aspx"));
                            

                            //Response.Write("<script>document.getElementById('iframe').src='login.aspx';</script>");
                            return;
                        }
                        break;
                    }
                }
            }
        }
        protected void Page_Load(object sender, EventArgs e)
		{

		}
	}

注意：关于继承，如下

 public partial class allNews : CommonPage

新建一个Global.asax，Session_End中添加如下代码

	protected void Session_End(object sender, EventArgs e) 
	{
            Hashtable hOnline = (Hashtable)Application["Online"];
            if (hOnline[Session.SessionID] != null)
            {
                hOnline.Remove(Session.SessionID);
                Application.Lock();
                Application["Online"] = hOnline;
                Application.UnLock();
            }
        }

原文链接：https://www.cnblogs.com/f23wangj/p/4984302.html