Why was ReadWriteVariable not populated/updated with a value of the dynamically generated string?

Hi, I have a script task in SSIS which dynamically generates a string for a file name to be used as flat file source. I execute the task and it executed with success; but when I checked the result of the variable TotFileName from the Expression builder window for the flat file connection manager it was not populated with a file name like \\MyServer\MyDrive\MyFolder\200706daily.txt. So something might still be missing from the script below. Or is the way I do it correct Can someone help with this Thanks a lot!!

I have created package level variables ImportFolder (value like: \\MyServer\MyDrive\MyFolder ) and TotFileName (value field empty) and make ImportFolder a ReadOnlyVariable and TotFileName a ReadWriteVariable. Then I use expression to set the property for flat file connection manager to use the TotFileName variable.

(Basically the idea is: if now is July 2007 then the filename should be 200706daily.txt; if now is Jan 2008 then the filename should be 200712daily.txt)

-----code------

' Microsoft SQL Server Integration Services Script Task
' Write scripts using Microsoft Visual Basic
' The ScriptMain class is the entry point of the Script Task.

Imports System
Imports System.IO
Imports System.Data
Imports System.Math
Imports Microsoft.SqlServer.Dts.Runtime

Public Class ScriptMain

' The execution engine calls this method when the task executes.
' To access the object model, use the Dts object. Connections, variables, events,
' and logging features are available as static members of the Dts class.
' Before returning from this method, set the value of Dts.TaskResult to indicate success or failure.
'
' To open Code and Text Editor Help, press F1.
' To open Object Browser, press Ctrl+Alt+J.

Public Sub Main()

Dim TotFileName As String
Dim TrueFileName As String
Dim sYear As String
Dim sMonth As String
Dim sDate As String

sYear = CStr(Year(Now()))
sMonth = CStr(Month(Now()) - 1)

If (Month(Now()) < 11 And Month(Now()) > 1) Then
sMonth = "0" & sMonth
End If

If (Month(Now()) = 1) Then
sMonth = "12"

sYear = CStr(Year(Now()) - 1)
End If


sDate = sYear & sMonth

TrueFileName = sDate & "daily.txt"

TotFileName = CStr(Dts.Variables.Item("ImportFolder").Value) & TrueFileName

Dts.Variables.Item("TotFileName").Value = TotFileName

Dts.TaskResult = Dts.Results.Success
End Sub

End Class