Friday, April 24, 2009

Simple reusable DTO factory methods

I have just found a little bug in my app that to me was an issue with code duplication. It was a DTO not getting properly hydrated when is was getting translated from a domain object in the service layer. My DTOs are just objects with auto properties, no business logic methods; just data carriers. I sometimes have the need for a basic DTO with just simple info (ie for Lists) and a more detailed DTO with the objects child collections (as DTOs collections) for more detailed views. The problem I had was when I added a field on to the domain object then had to modify my factory (and test) to ensure the new field was mapped. What I forgot to do was to also do it for my detailed DTO. I wtote the test and realised that I was doing the exact same work in 2 places, which was one of the reason behind the bug. As i prefer not to use anything other than a default constructor for DTOs i was in a bit of a quandry. Other than using JBogard's AutoMapper i was not sure how to tackle this.
My goodness, i may actually have to engage my brain!
Well the result was incredibly simple.
The detailed DTO (xyzDetailedDto) inherited from the normal DTO (xyzDto) so i just created a private generic factory method in the publicly exposed Extension method class

private static T Create(xyzDomianObject domainObj) where T : xyzDto, new()
return new T
Id = domainObj.Id,
Name = domainObj.Name,
Details = domainObj.Details,
OtherThing = domainObj.OtherThing

As xyzDetailedDto inherited from xyzDto I could reuse the creation method and get back the correct type.
This all seems very simple and silly now, but this has drastically cleaned up my translation layer :)

No comments: