I am currently working with model validation in ASP.NET, using an Entity Framework 4 entity model, DataAnnotations and partial classes with MetadataType connections.

In my model, I have an Employee entity for which some properties are non-nullable. I have also created a partial class and a meta data class for this class, to be used with model validation, as described in this blog post.

This approach works great. The Employee class is validated properly, with a minimum effort. My entities are validated with standard validation attributes, as well as a set of custom ones. Lovely.

However, the application crashes when I post empty text input elements in the Create and Edit views. A ConstraintException is thrown before any view controller actions are executed, which means that I cannot act on the constraint exception in my action.

The exception occurs since the empty posted data causes the corresponding properties on the Employee class to be set to null, which conflicts with the non-null setup in the entity model.

However, since I have custom validation classes where I use Required attributes to make properties mandatory, I do not need the non-nullable attributes in my model. As such, I set the nullable property to (None)…and the ConstraintException is history!