Validate Start Date < End Date in SSRS 2008

HI

I have used the same Code but will not get the message box while accessing.

step1)

Function CheckDateParameters(@StartDate as Datetime, @EndDate as Datetime) as Integer
Dim msg as String
msg = ""
If (@StartDate > @EndDate) Then
msg="Start Date should not be later than End Date"
End If
If msg <> "" Then
MsgBox(msg, 16, "Report Validation")
Err.Raise(6,Report) 'Raise an overflow
End If
End Function

step2)

created a hidden parameter "@validate" and set its default to
=CODE.CheckDateParameters(@StartDate.Value,@EndDate.Value)

Please help I just want to validate the Start Date < Endate in SSRS 2008 before clicking the View report button.

SD