Making Values Required in a Silverlight DataForm

For those of you who have done validation in a Silverlight DataForm before, you probably are used to setting the [Required] attribute above all of the necessary fields in your WCF RIA project. From there, .NET will auto-magic the necessary code and, combined with lots of NotifyOnValidationError and ValidatesOnExceptions tags (and potentially some GetBindingExpression().UpdateSource()), viola! it will successfully perform validations. However, I ran into a situation where I was actually taking a couple of fields and transforming them into a single field from my WCF RIA service data object. After a long time of scratching my head, I was really stuck on trying to figure out how to force this validation to work. I had tried setting the [Required] attribute on the field they were bound to, added all the usual NotifyOnValidationError tags... nothing.

Eventually, I realized how to get around this issue (and it made me feel a bit silly for not coming up with this earlier.) You know where we set "ValidatesOnExceptions=true"... well... let's throw a ValidationException! So, in the setter of the property that I was binding the DataField to, if the data was invalid, I simply threw a ValidationException. Poof! It suddenly started working. Is there a better way to do this? Probably. However, this was able to get me back on the right path, and I was able to resume being productive.