title: Business Objects, Validation And Exceptions
body: <p>I’ve been reading a few questions and answers regarding exceptions and their use. Seems to be a strong opinion that exceptions should be raised only for exception, unhandled cases. So that lead me to wondering how validation works with business objects.</p>
  
  <p>Lets say I have a business object with getters/setters for the properties on the object. Let’s say I need to validate that the  value is between 10 and 20. This is a business rule so it belongs in my business object. So that seems to imply to me that the validation code goes in my setter. Now I have my UI databound to the properties of the data object. The user enters 5, so the rule needs to fail and the user is not allowed to move out of the textbox. . The UI is databound to the property so the setter is going to be called, rule checked and failed. If I raised an exception from my business object to say the rule failed, the UI would pick that up. But that seems to go against the preferred usage for exceptions. Given that it’s a setter, you aren’t really going to have a  ‘result’ for the setter. If I set another flag on the object then that would imply the UI has to check that flag after each UI interaction.</p>
  
  <p>So how should the validation work?</p>
  
  <p>Edit: I've probably used an over-simplified example here. Something like the range check above could be handled easily by the UI but what if the valdation was more complicated, e.g. the business object calculates a number based on the input and if that calculated number is out of range it should be recjected. This is more complicated logic that should not be in th UI. </p>
  
  <p>There is also the consideration of further data entered based on a field already entered. e.g.I have to enter an item on the order to get certain informaion like stock on hand, current cost, etc. The user may require this information to make decisions on further entry (liek how many units to order) or it may be required in order for further validation to be done. Should a user be able to enter other fields if the item isn't valid? What would be the point?</p>