Get Current Online User Count

I am attempting to get a count of the users currently on my site.  Right now I have the following code implemented:
In global.asax

 Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        Application("ActiveUsers") = 0
    End Sub
   Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
        Application.Lock()
        Application("ActiveUsers") = CInt(Application("ActiveUsers")) + 1
        Application.UnLock()
     
    End Sub
  Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
        Application.Lock()
        Application("ActiveUser ...