Class consultant response; "It Depends".

Really, it depends on what the rules are.

One such rule might be (this is from a real spec):  "Display the current
season first followed by the next 2 future seasons in ascending order.
Finally, append all other seasons in reverse chronological order".  This
rule can be met using TSQL (although it took me 10 minutes just to code the
sql for a parameter query).

Really complex business rules may best be written in C# and handled outside
of the report, then passed to it sans parameter toolbar.  I honestly haven't
come across any that I couldn't handle in TSQL though.  Can you give me an
example of some you've dealt with?

If they're dropdowns, the user can't key in an invalid date.

As an aside, you could also use custom code to perform parameter validation
although this is a real pain in the posterior and you may spend more time
working around that than actually building a good report, but,

You ~could~ have a textbox above your report visualization (i.e., Table,
Matrix, List) and add 2 methods to the report in the code section:

' assume you have date1 and date2 as parameters

Public shared function IsValidParameterList(date1 as string, date2 as
string) as boolean
   dim startDate as dateTme = datetime.parse(date1)
   dim endDate as dateTime = dateime.Parse(date2)
   if ( endDate < startDate) then
       return false
   end if
   return true
End function

Public shared function GetInvalidParameterMessage(date1 as string, date2 as
string) as string
   Dim rtnvalue as string
   dim startDate as dateTme = datetime.parse(date1)
   dim endDate as dateTime = dateime.Parse(date2)
   if ( endDate < startDate) then
       rtnvalue = "End date must be greater than start date"
   end if
   return rtnvalue
End function

Then in your report, the textbox would have visibility expression =
"=(Code.IsValidParameter(Parameters!StartDate.Value.ToString(),
Parameters!EndDate.Value.ToString()))"

And your report visualization would have visibility expression = "=(NOT
Code.IsValidParameter(Parameters!StartDate.Value.ToString(),
Parameters!EndDate.Value.ToString()))"

This is purely an academic exercise.  Like I said, you could spend almost as
much time doing validation like this as you could giving the user a good
report and an hour of training on how to use it.  At the end of the day, you
really can't protect a user from themselves but so much.  It's cost
prohibitive.

-Tim