using Err.Raise in SSRS 2008

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reporting-services

Hi bro !!!
Now i want to custom validate for some field by using Err.Raise.
I code in code tab(report property):

Public Function CheckDate(StartDate as Date, EndDate as Date) as Integer
Dim msg as String
                msg = ""
                If (StartDate > EndDate) Then
                                  msg="Start Date should not be later than End Date"
                                Err.Raise(22000, "VBCore.Utility",msg)
                End If

End Function

I add a parameter X with default value expression :
=Code.CheckDate(Parameters!StartDate.Value,Parameters!EndDate.Value)

But when i run report, it displays
"The defaultvalue expression for the parameter "X" contains error: Start Date should not be later than End Date"

Is it right ???
I want to display only : "Start Date should not be later than End Date".
Is it possible ???
if i dont add default value for parameter X can i run function CheckDate when run report ???