Opinion on ASP.NET MVC Onion-based architecture

In my opinion:

• "In classic Onion architecture, the core is referenced only by the next layer." That is not true, Core should be reference by any layer... remember that Direction of dependency between layers is toward the center (Core).

"the layers above can use any layer beneath them" By Jeffrey Palermo http://jeffreypalermo.com/blog/the-onion-architecture-part-3/

• About your EF, it is in the wrong place.... it should be in the Infrastructure layer not in the Core layer. And use POCO Generator to create the entities (POCO classes) in Core/Model. Or use a Auto-mapper so you can map Core Model (Business objects) to Entity Model (EF Entities)

I inject my services into my controllers. The services return DTO's which reside in Core. The model you have looks good, I don't use the repository pattern but many people do. I is difficult to work with EF in this type of architecture which is why I chose to use Nhibernate.

A possible answer to your final question.

1. CORE
2. DOMAIN
3. DI
4. INFRASTRUCTURE
5. PRESENTATION
6. SERVICES

Wow, there’s a lot to say here! ;-)

First of all, let’s talk about the overall architecture.

What I can see here is that it’s not really an Onion architecture. You forgot the outermost layer, the “Dependency Resolution” layer. In an Onion architecture, it’s up to this layer to wires up Core interfaces to Infrastructure implementations (where your Persistence project should reside).

Here’s a brief description of what you should find in an Onion application. What goes in the Core layer is everything unique to the business: Domain model, business workflows... This layer defines all technical implementation needs as interfaces (i.e.: repositories’ interfaces, logging interfaces, session’s interfaces …). The Core layer cannot reference any external libraries and has no technology specific code. The second layer is the Infrastructure layer. This layer provides implementations for non-business Core interfaces. This is where you call your DB, your web services … You can reference any external libraries you need to provide implementations, deploy as many nugget packages as you want :-). The third layer is your UI, well you know what to put in there ;-) And the latest layer, it’s the Dependency Resolution I talked about above.

Direction of dependency between layers is toward the center.

Here’s how it could looks like:

The question now is: how to fit what you’ve already coded in an Onion architecture.

Core: contain the Domain model

Yes, this is the right place!

Persistence - Repository interface and implementations

Well, you’ll need to separate interfaces with implementations. Interfaces need to be moved into Core and implementations need to be moved into Infrastructure folder (you can call this project Persistence).

BusinessServices - A business layer around the repository. All the business logic should be here

This needs to be moved in Core, but you shouldn’t use repositories implementations here, just manipulate interfaces!

Web (or any other UI) - In this particular case it's an MVC application

cool :-)

You will need to add a “Bootstrapper“ project, just have a look here to see how to proceed.

About your mixed feelings:

I won’t discuss about the need of having repositories or not, you’ll find plenty of answers on stackoverflow.

In my ViewModel project I have a folder called “Builder”. It’s up to my Builders to discuss with my Business services interfaces in order to get data. The builder will receive lists of Core.Domain objects and will map them into the right ViewModel.

About what you don’t like:

In classic Onion architecture, the core is referenced only by the next layer.

False ! :-) Every layer needs the Core to have access to all the interfaces defined in there.

The DbContext is implemented in the .Core project, because it is being generated by the Entity Framework, in the same place where the .edmx is

Once again, it’s not a problem as soon as it’s really easy to edit the T4 template associated with your EDMX. You just need to change the path of the generated files and you can have the EDMX in the Infrastructure layer and the POCO’s in your Core.Domain project.

Hope this helps!