I don't think it is a good idea to create dll for asp.net control validation.
It is always recommended to do validation at client side otherwise performance of the application will be very poor.
If you are very particular about dll, then create a class file with all required validation as seperate funtion and call the funtion by passing the control name when you want to validate the same.
Still this is not going to be a good way of programming...
Asheej T K