how to clear a session after logout

In addition to the above solution, you should use

Session.Abandon()

on the Page_Load event of logout page. Read more here http://msdn.microsoft.com/en-us/library/ms524310(v=vs.90).aspx[^]

These three methods are used to clear session variables from the server.

***Session.Abandon includes Session.RemoveAll and also deletes the session id created for the session. So, subsequent request to .aspx page would create a new session with a new session id.

***Session.RemoveAll will remove all the user defined session variables but still keeps the session active.so if u want to end session you must use Session.Abandon().

***Session.Clear() do the same job as Session.RemoveAll().

You can refer to wonderful explanation on below link:

What is the difference between Session.Abandon() and Session.Clear()

It states that:

Clear - Removes all keys and values from the session-state collection.

Abandon - removes all the objects stored in a Session. If you do not call the Abandon method explicitly, the server removes these objects and destroys the session when the session times out.
It also raises events like Session_End.

Session.Clear can be compared to removing all books from the shelf, while Session.Abandon is more like throwing away the whole shelf.

Both the answers above are correct,its totally up to you how you want your session to be handled.

just use this code

session.clear();
session.abandon();

iot will fix your problem

Session.Clear();
session.abandon();
or
session["User"] = null;

There are three Ways:

1) session.clear();
2) session.abandon();
3) session["sessionname"] = null; e.g: session["UserID"] = null;

on Login_Page write following code on page load

protected void Page_Load(object sender, EventArgs e)
        {
             Session.Remove("ABC");
        }