I am trying to get a handle on how to practically develop an 3-tier application design, and wonder how others have tackled user communications. Assuming the business services layer contains the rules, how do you communicate to the user if a rule has been violated ? I understand that the middle layer (business services) should not have any UI, so if I want to display a messagebox that tells the user the rule was violated, how do I do it ?

I am assuming the following design - a form has a method called Save(), which calls the Save() method of the associated business service object (oBusObject). If the oBusObject.Save() method fails, how do I present this to the user ? I originally thought the form should display the message, but wonder how it would know what to say ????

Anyway, has anyone already tackled this and can they offer some insight ?

Thanks in advance
Rob