If you read this article on Validation with the Data Annotation Validators, it shows that you can use the MetadataType attribute to add validation attributes to properties on partial classes. You use this when working with ORMs like LINQ to SQL, Entity Framework, or Subsonic. Then you can use the "automagic" client and server side validation. It plays very nicely with MVC.

However, a colleague of mine used an interface to accomplish exactly the same result. it looks almost exactly the same, and functionally accomplishes the same thing. So instead of doing this:

[MetadataType(typeof(MovieMetaData))]
public partial class Movie
{
}

public class MovieMetaData
{
[Required]
public object Title { get; set; }

[Required]
[StringLength(5)]
public object Director { get; set; }


[DisplayName("Date Released")]
[Required]
public object DateReleased { get; set; }
}

He did this:

public partial class Movie :IMovie
{
}

public interface IMovie
{
[Required]
object Title { get; set; }

[Required]
[StringLength(5)]
object Director { get; set; }


[DisplayName("Date Released")]
[Required]
object DateReleased { get; set; }
}

So my question is, when does this difference actually matter?

My thoughts are that interfaces tend to be more "reusable", and that making one for just a single class doesn't make that much sense. You could also argue that you could design your classes and interfaces in a way that allows you to use interfaces on multiple objects, but I feel like that is trying to fit your models into something else, when they should really stand on their own. What do you think?