AOP, DataMappers, and Factories, can they work together?

Question

Platform: C# 2.0 Using: Castle.DynamicProxy2

I have been struggling for about a week now trying to find a good strategy to rewrite my DAL. I tried NHibernate and, unfortunately, it was not a good fit for my project. So, I have come up with this interaction thus far:

I first start with registering my DTO's and my data mappers:

MetaDataMapper.RegisterTable(typeof(User)):
MapperLocator.RegisterMapper(typeof(User), typeof(UserMapper));

This maps each DTO as it is registered using custom attributes on the properties of the DTO essentially:

[Column(Name = "UserName")]

I then have a Mapper that belongs to each DTO, so for this type it would be UserMapper. This data mapper handles calling my ADO.Net wrapper and then mapping the result to the DTO. I however am in the process of enabling deep loading and subsequently lazy loading and thus where I am stuck. Basically my User DTO may have an Address object (FK) which requires another mapper to populate that object but I have to determine to use the AddressMapper at run time.

My problem is handling the types without having to explicitly go through a list of them (not to mention the headache of always having to keep that list updated) each time I need to determine which mapper to return. So my solution was having a MapperLocator class that I register with (as above) and return an IDataMapper interface that all of my data mappers implement. Then I can just cast it to type UserMapper if I am dealing with User objects. This however is not so easy when I am trying to determine the type of Data Mapper to return during run time. Since generics have to know what they are at compile time, using AOP and then passing in the type at run time is not an option without using reflection. I am already doing a fair bit of reflection when I am mapping the DTO's to the table, reading attributes and such. Plus my MapperFactory uses reflection to instantiate the proper data mapper. So I am trying to do this without reflection to keep those expensive calls down as much as possible.

I thought the solution could be found in passing around an interface, but I have yet to be able to implement that idea. So then I thought the solution would possibly be in using delegates, but I have no idea how to implement that idea either. So...frankly...I am lost, can you help, please?

Answer 1

I will suggest a couple of things.

1) Don't prematurely optimize. If you need to use reflection to instantiate your *Mappers, do it with reflection. Look at the headache you're causing yourself by not doing it that way. If you have problems later, than profile them to see if there's faster ways of doing it.

2) My question to you would be, why are you trying to implement your own DAL framework? You say that NHibernate isn't a good fit, but you don't elaborate on that. Have you tried any of the dozens of other ORM's? What's your criteria? Your posted code looks remarkably like Linq2Sql's mapping.

Lightspeed and SubSonic are both great lightweight ORM packages. Linq2Sql is an easy-to-use mapper, and of course there's Microsoft's Entity Framework, which has a whole team at Microsoft dealing with the problems you're describing.

You might save yourself a lot of time, especially maintenance wise, by looking at these rather than implementing it yourself. I would highly recommend any of those that I mentioned.