c# how to set up and use session state from preinit

Question

OK so to set and read variables from the current session

String Myvar =(string) System.Web.HttpContext.Current.Session[“MyVariable”]

To set

Answer 1

As the comment mentioned, is there some reason you need this in the PreInit event?

PreInit happens very early in the page lifecycle. It happens, in fact, before even the master page (if any) is applied, before all the controls are fully initialized, etc.

A much better choice for the majority of applications is in the Load event. If you're still getting a NullReferenceException there, then there's a bigger issue.