Thinking about this, in thread Peter linked, Lukas mentions custom form control which would work on administration backend, but on MVC application it will not work. I guess we need a bit more context in here. Simple [A] , you are mentioning that you are using MVC. Are you trying to use handler in MVC app? If it is in CMS what Peter posted is perfectly valid, but in MVC app, why not use controller action to validate data and cancel event.