change user status to offline when session automatically expires

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C#

ASP.NET

hi!
i want to change the status of user when the session automatically expires.i have a table user with field status.when the user logins i change its status to online and when the user log off the status is changed to offline.
but when the session automatically expires the status remains online ...i want to change its status to offline..i know in page load i can check when session["username"]==null then in that block i will write the code..but i need the value of session variable..so that i pass it to the function and change the status against that username....is there any way to store that value of session variable some where else so that when session expires even then i could get the value of session variable