Combine DataAnnotations Validation with complex business rules

Question

I understand annotating class properties with the basic required and minimum length and getting all the benefits of the asp.net mvc server side and client side validation.

However does anyone have a link that shows how you combine this 'base' validation with more complex business rules. How would I run business rule functions, such as for example, has the customer ordered anything in the last year (database hit required) and still use the same DataAnnotation and mvc validation plumbing?

Goal : Don't want two ways of generating and outputting validation methods.

Answer 1

Data Annotation run before your action is invoked. Then, regardless whether the validation succeded or not, the action is still called. If the DA detected invalid data, your ModelState will be invalid.

Once here, you can still do any validation you want, for your business rules, as you would normally do without the data annotation, if you want to. In your action, you can add errors to the ModelState even if the Data Annotation validation passed. In this case, you add your errors with ModelState.addError, and those errors are added to any error provided by the DA. So in your View it doesn't matter where the error comes from.

Or, if your rules are general, you can write your own annotation tags. The Data Annotation thing is distributed with its source, so you have full control on it.

Answer 2

From http://msdn.microsoft.com/en-us/library/dd901590%28VS.95%29.aspx:

To create customized validation checks, you can either create a class that derives from the ValidationAttribute class or create a method that performs the validation check and reference that method when applying the CustomValidationAttribute to the data member. When you create a class that derives from ValidationAttribute, override the IsValid method to provide the logic for your customized validation check.

There appears to be example code there.