Session check in login page

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ASP.NET

Hi,
the logout button clear the session using the code below and then redirect to login
page.

FormsAuthentication.SignOut();
            Session.Clear();
            Session.Abandon();

Then, each page call a function ValidateSession() in a class checkLogIn as
CheckLogIn.ValidateSession() in page load event.

the function ValidateSession() as below,

public static void ValidateSession()
        {
            try
            {
                string sUserId = System.Web.HttpContext.Current.Session["UserId"].ToString();
            }
            catch (Exception oException)
            {
                System.Web.HttpContext.Current.Session["UserId"] = "sessionOut";
                System.Web.HttpContext.Current.Response.Redirect("~/Account/Login.aspx");

            }
            
        }

Then, check the session, Session["LoginId"] in page load event of login page.
as below

if (Session["UserId"] != null)
           {
               ScriptManager.RegisterStartupScript(this, this.GetType(), "alertscript", "find()", true);
           }

But the Session["UserId"]==null.why?

Thanks..