I've got a C# MVC 2 application using EF 4. My DAL project gets data from the Model, translates them into POCO's and returns ViewModels to the BLL which is called from the controllers. I've set up validation attributes on the POCO classes and that works for "Required" fields etc.
I would like to add business logic such as "From" has to be before the "To" date. Can this be done on the POCO classes so it is automatically validated client-side in the view?
Also, I would like to add some functionality to the BLL layer such as checking if a resource is already booked between the From and To dates to avoid double bookings. This should fire before the ViewModel is saved.
I tried a Try/Catch statement in the controller so that I can throw an error in the BLL, but I don't understand how to return this error to the view with an appropriate error message added.