You can use
object for that, see here
Now, at the log in time, store your user name/ user id in one Session object like
Session["Username"] = UserNameTxt.Text;
Session.Count > 0
, so you can use following scenario at Searching Chart:
if(Session.Count > 0)
Hope it will solve your issue..
If there is something that you don't get about this, let me know...