problem in !ispostback

I have problem with !IspostBack i'm working with crystal report and i use (if not ispostback) to set X and in else to retrieve X the problem is its working properly in some pages and not working in others. this code sample.

If Not Page.IsPostBack Then
     Session("VehicleStatusSelectionFormula") = SelectionFormula
Else
     SelectionFormula = Session("VehicleStatusSelectionFormula")
End If

note: is ("EnableViewStateMac="false") make if not ispostba开发者_StackOverflowck doesn't work ? beacause the page that doesn't work has this in page register

<%@ Page Language="vb" AutoEventWireup="false" CodeFile="ReportsVehicleStatusScript.aspx.vb" Inherits="ReportsVehicleStatusScript" **EnableViewStateMac="false"**  %>

please i need some help why its working in some pages and not in others ?

thanks.

finally I found the problem : the problem like i said in last note with ("EnableViewStateMac="false") it make ispostback true all the time.

finally I found the problem : the problem like i said in last note with ("EnableViewStateMac="false") it make ispostback true all the time