Control Property Refresh

I Created 3 Properties to my Control Mindate(retuns date) MaxDate ( return date )
and a CustomList Property (allDates,Uptodate...)
when i choose from the list allDates for example the Mindate and MaxDate are set to the values i set in the method for the CustomList,
The thing is that in Design time when i select allDates i don't see the changes on minDate and maxDate, only when i click on them(on the property box) they change to the currect date,
How can i make them change immidiatly in Design time when i select allDates etc,,,,

Thanks

Tomer