MVVM with WPF using LINQtoSQL in a DAL along with a BLL

Question

My goal is to have an app that is using WPF and is a 3 tier architecture. UI, BLL, and DAL...I'd like to use the MVVM but I'm not sure how that works with a 3 tier architecture or if it is something entirely different. So with that in mind, I have a few questions:

1) LINQtoSQL: I've read a lot online that say LINQ replaces your DAL and seen many articles that say this a bad idea. I'm thinking it's a bad idea, however, what do I put in here? What are the datatypes i am returning to the BLL? IQueryable? ObservableCollection? I have no clue.

2) The BLL: I'd like to make this a service that runs on a server, so that when I need to make a change I don't need to redeploy the whole app, I just need to restart the service. But, I'm not sure where to start on this.

3) With the BLL, I guess I'm confused on how the data is going through all the layers from the DAL all the way to the Interface.

I've done lots of research online, and have got bits and pieces of things, however I haven't seen anyone talk about a WPF application that is using MVVM with LINQ in the DAL using SQLMetal and a BLL thats running on a server. Can anyone point me in the right direction? or maybe a book to get?

Answer 1

Mike,

your question is really cool, I like it. Firstly, feel free to experiment a bit - each and every project is different, so there's no single rule, which fits everywhere. That's why I would suggest just leaving DAL to LINQ 2 SQL. this great tool will handle it, you don't have to worry. Secondly - you mentioned 3 Tier Architecture, but why isn't there place for the Model? Since all models are generated automatically (e.g. SQLMetal), you don't have to worry about mappings either. So, if you're not bored yet, let me answer all of your 3 questions:

1. Skip DAL and observe your project carefuly - if you have a feeling, that it's lacking this layer - add it (it will contain LINQ2SQL queries). And the second part - you can return whatever you wish, but it will be most convenient for you to use IEnumerable<> or IQueryable<> parametrized with your models.

2. My intuition tells me, that you're going to need WCF - in this case you should be able to wrap whole (yes, that's true) whole business logic in a nice Contract and implement however you wish.

3. This is the easiest one :) Since your BLL layer is actually an implementation of some Contract (Interface), you can design that Interface to provide you with all data you need. For example:

Contract/Interface:

IEnumerable<User> GetTallUsersOver40();
IEnumerable<User> GetShortUsersOver60();
...

And that 'all the layers' you were talking about shrink to a single LINQ2SQL query execution. If you need more logic - place it in this layer.

I want to use MVVM, what now? The answer is simpler than you think - just prepare your views and view models and simply consume your BLL Contract/Interface implementaion.

Please ask if you have further questions!

Answer 2

I'll try to provide some insight, though I'm not an expert, I've tackled these issues in the past.

1. LINQ to SQL is actually pretty good at what it's supposed to do - which is replace your DAL. But don't return an IQueriable upwards to your BLL, as that would enable (or at least hint to the possibility) the BLL to query your DB directly. You should transfer the data object to the BLL and let it construct a matching business object. Also note: LINQ in and of itself can be used in any layer (and in fact is one of the best features of C#). LINQ to SQL is the mechanism by which LINQ statements get transated to SQL queries.

2. BLL as a service is a natural choice. Provide an upward interface to the presentation layer (a WCF service is a good option here).

3. The BLL generates business objects based on data it receives from the DAL. In order to provide for good decoupling of layers, you should use different classes for your DAL and BLL objects. Don't create a dependency between your presentation layer and your data layer.