Session_OnEnd Event Not Firing

We currently have a web app that times out after 20 minutes as intended. When this occurs I want to display a message box telling the user that the session has timed and out an to click OK to refresh the page.

I have the following VB code on the back end:

Imports System.Windows.Forms

Partial Class _Default
    Inherits System.Web.UI.Page
    Public Event OnEnd As EventHandler
    Public Property Timeout As Integer

    Public Sub Session_OnEnd()
        MessageBox.Show("Your session has timed out. Click OK to refresh your page.", _
            "Important:", _
             MessageBoxButtons.OK, _
             MessageBoxIcon.Exclamation, _
             MessageBoxDefaultButton.Button1)

        Dim result As Integer = MessageBox.Show("message", "caption", MessageBoxButtons.OK)
        If result = DialogResult.OK Then
            Response.Redirect("default.aspx")
        End If
    End Sub
End Class

And for testing purposes I’ve set the following entries in the web.config file to 1 minute for testing:

<sessionState mode="InProc" cookieless="false" timeout="1"/>

<authentication mode="Forms">
      <forms loginUrl="~/Account/Login.aspx" timeout="1"/>


The problem is that the event doesn’t seem to fire. I’m going into debug, executing one search and leaving the page alone, the nothing is happening. What am I missing here?

Thanks,

JB