Why are buddy classes used for validation?

Question

I am curious as to why data validation is done using buddy classes. Consider the following example, where MyEntity is a Linq-to-SQL or Linq-to-Entities entity, and the class below is a partial class enhancing the entity.

[MetadataType(typeof(MyEntity.MyEntityMetadata))]
public partial class MyEntity
{
    private class MyEntityMetadata
    {
        [Required(ErrorMessage = "The title is required.")]
        public string Title { get; set; }
    }
}

Why is the design so? When they designed DataAnnotations, why was this "buddy pattern" selected? Why not place the attributes directly in the entity?

Answer 1

I assume this prevents generated entities from overwriting custom Meta Data information.

Answer 2

The reason is practical - in linq-to-sql and linq-to-entities, the code representing the classes regenerated every time the object model is updated. In order for the annotations not to be overwritten when this happens, they need to be in a separate "buddy" class.

If you're using Data Annotations in a different context - say for a view model - then they can go on the original class itself.