I am creating a custom datatype using the usercontrol wrapper pattern to store ids that need to be validated against an external service before being saved. If the validation fails then they shouldn't be saved and a message should appear. Is it possible to cancel the save from within a custom datatype's codebehind during the postback caused by saving the document? If not is there a standard way to alert the user that the data entered for this particular datatype was not saved due to validation issues?