Wrecker,
Yes....but there are a number of ambiguities associated with your question. Is the Textbox bound or unbound? If it is unbound, then of course you will handle all data-validation before you access your db, this would be a great example of a "Class", ie you create a Class to handle your interface with your db. If you are using Bound Controls, then you should probably put your data validation code in the binding object's (DataControl) validatedata event; however, as long as the data is validated before it is saved then we are only really argueing symantics.
Hunter