Repository; Mapping between POCOs / Linq-to-Sql entity classes

Question

I'm making my first database program, with Sql Express. Currently I'm using Linq-to-Sql for data access, and my repository classes return "entity" type objects. Meaning; I extend the dbml entity classes to use as my business object classes. Now I want to make this more separated; and have POCO bussiness objects.

This is where I wonder about what different solutions may exist. It looks to me like I need to manually map property-by-property, each entity class into domain class, in the repositories. I have so far about 20 tables with total few hundred columns. Now.. I just want to verify if this is a common/typical approach that you still use? And if there are alternatives without introducing excessive complexity, what would that be?

Answer 1

Before creating your mappings manually, have a look at AutoMapper

AutoMapper is an object-object mapper. Object-object mapping works by transforming an input object of one type into an output object of a different type. What makes AutoMapper interesting is that it provides some interesting conventions to take the dirty work out of figuring out how to map type A to type B. As long as type B follows AutoMapper's established convention, almost zero configuration is needed to map two types.

Answer 2

AutoMapper is a good tool to use to perform class-to-class conversions. However, I'm thinking of a DAL that combines Linq2Sql and AutoMapper, and I'm thinking why not just go with Fluent NHibernate? It's very easy to set up, works on just about any database including SqlExpress, and there is a Linq provider that integrates pretty seamlessly. All of this is free open-source code, and very commonly-used so there's ample documentation and support.

If you want to stay with Linq2Sql but have a more full-featured domain model, you could consider deriving your domain model from the DTOs. That would allow you to have the business logic in the domain, with the properties passed up to the DTO. However, understand that the Linq2SQL objects will not be able to be directly cast to domain objects; you'll need a constructor in the domain that takes a DTO and copies the info into the domain (requiring at least a one-way mapping of DTO to domain). However, the domain can be treated like a DTO (because a class is always its parent) so the reverse conversion isn't necessary; just hand the domain class to the repository where it would expect the DTO.