Mapping Validation Attributes From Domain Entity to DTO

If you're using something supporting DataAnnotations, you should be able to use a metadata class to contain your validation attributes:

public class ProductMetadata 
{
    [NotEmpty, NotShorterThan10Characters, NotLongerThan100Characters]
    public string Name { get; set; }

    [NotLessThan0]
    public decimal Price { get; set;}
}

and add it in the MetadataTypeAttribute on both the domain entity & DTO:

[MetadataType(typeof(ProductMetadata))]
public class Product

and

[MetadataType(typeof(ProductMetadata))]
public class ProductViewModel

This won't work out of the box with all validators - you may need to extend your validation framework of choice to implement a similar approach.

The purpose of validation is to ensure that data coming into your application meets certain criteria, with that in mind, the only place it makes sense to validate property constraints, like those you have identified here, is at the point where you accept data from an untrusted source ( i.e. the user ).

You can use something like the "money pattern" to elevate validation into your domain type system and use these domain types in the view model where it makes sense. If you have more complex validation (i.e. you are expressing business rules that require greater knowledge than that expressed in a single property), these belong in methods on the domain model that apply the changes.

In short, put data validation attributes on your view models and leave them off your domain models.

It turns out that AutoMapper may be able to do this for us automagically, which is the best case scenario.

AutoMapper-users: Transfer validation attributes to the viewmodel?
http://groups.google.com/group/automapper-users/browse_thread/thread/efa1d551e498311c/db4e7f6c93a77302?lnk=gst&q=validation#db4e7f6c93a77302

I haven't got around to trying out the proposed solutions there, but intend to shortly.

Why not use an interface to express your intent? Eg:

public interface IProductValidationAttributes {
    [NotEmpty, NotShorterThan10Characters, NotLongerThan100Characters]
    string Name { get; set; }

    [NotLessThan0]
    decimal Price { get; set;}
}