My current project is organized in this way:
Domain Layer -> Domain objects and Repository Interfaces
Infrastructure -> Repository implementation
Application Layer -> Services in a MVVM pattern
Presentation -> Accessing only Service Layer and working with ViewModels
I´m using an IoC Container (SimpleInjector). My Services receives an IRepository in the constructor like:
public CustomerService : ServiceBase
{
public CustomerService(ICustomerRepository repository, IUnitOfWork<CustomerContext> uow)
{
...
}
}
My question is:
To inject a Repository in the Service, my Presentation Layer should reference the Domain Layer. It´s ok to add this reference? Shouldn't my presentation layer have reference only to the Infrastructure and the Application layer?
Yes, that's OK.
You need a reference to all components from the composition root (which usually resides in the presentation layer).
I understand it feels a bit strange at first, but you need to differentiate between a DLL-dependency and a hard class dependency. It's OK if your presentation layer depends on DLL's, it's not OK when a view depends on a SQL-repository (as an example).
I have written a blog post about this with some more information:
http://www.kenneth-truyers.net/2013/05/12/the-n-layer-myth-and-basic-dependency-injection/
Related
I have 3 projects in .NET
DAL (data access layer)
UI (user interface)
BI ( logic)
DAL call the BI (services)
And The BI Call the DAL (repository)
my problem in my UI project in (global.asax)
- I call my injector to Register my classes
public Application_Start()
{
......
container.Register<IService1, Service1Impl>();
container.Register<IService2, Service2Impl>();
}
But How I can Register my DAL Class without call them in UI Project
Else I have to add my in DAL project in My UI project
The Application_Start is not part of User Interface layer. The Application_Start is part of the Composition Root, which is a layer of its own. You only implicitly decided to place both the Composition Root and the User Interface Layer in the same assembly, which is fine.
For more details, about this, see this article: Understanding the Composition Root.
You chose onion-architecture and input-point is UI project. Your UI project must have links to DAL and to BI projects.
If you want to create independent modules in your application, you can choose microservices architecture.
I am a bit new to n-layer architecture and learning it by implementing a simple console app.
I have 3 projects :
DAL with Domain entities and DbContext class.
BLL with Repository class.
Console Application just to run it.
As all my Entities defined in DAL, BLL layer has reference to DAL and looks like:
public class DefaultRepository
{
private DefaultDbContext _repository;
private void SaveChanges()
{
try
{
_repository.SaveChanges();
}
catch (Exception e)
{
Console.WriteLine("Exception were caught");
Console.WriteLine(e.Message);
}
}
public void AddPatient(Patient patient)
{
_repository.Patients.Add(patient);
SaveChanges();
}
public Patient GetPatientById(int id)
=> _repository.Patients.Find(id) ?? null;
public void AddVisit(int patientId, Visit visit)
{
GetPatientById(patientId)?.Visits.Add(visit);
SaveChanges();
}
public DefaultRepository()
{
_repository = new DefaultDbContext();
}
}
The obvious problem is that I cannot use repository in my console application project because console application has no reference to DAL level. The following code occurs compile-time exception.
DefaultRepository repository = new DefaultRepository();
repository.AddPatient(new Patient());
Of course, I can solve it just by adding reference to DAL in ConsoleApplication project. However, I understand, that absolutely destroy the n-layer conception.
So, how should I manage this problem? I googled something about using auto-mappers...
Here I'm not directly giving solution to your use case.But I would like to give right path to implement sophisticated NLayer Architecture application.
What is NLayer Architecture
Layering of an application's codebase is a widely accepted technique
to help reduce complexity and improve code reusability. To achieve
layered architecture, we can follows the principles of
Domain Driven Design. In Domain Driven Design there are four
fundamental layers:
Presentation Layer: Provides an interface to the user. Uses the
Application Layer to achieve user interactions.
Application Layer: Mediates between the Presentation and Domain
Layers. Orchestrates business objects to perform specific application
tasks.
Domain Layer: Includes business objects and their rules. This is heart
of the application.
Infrastructure Layer: Provides generic technical capabilities that
support higher layers. An example of the Infrastructure Layer can be a
Repository implementation used to interact with a database through an
ORM framework, or an implementation for an email provider to send
emails.
There may be additional layers added as necessary. An example being:
Distributed Services Layer: Used to expose application features to
remote clients. There are tools like ASP.NET Web API and WCF that can
provide this layer. These are all common layers of a domain-centric
architecture. There may be minor differences based on implementation.
Overview of layers and structures are shown below:
Here is a solution with five projects for a simple layered application:
If you would like to learn more about this,I highly recommend to study the below mentioned project.It's free and opensource.
ASP.NET Boilerplate
From wiki (https://en.wikipedia.org/wiki/Multitier_architecture):
DAL "Encapsulates the persistence mechanisms and exposes the data".
Patient class is not a part of DAL but is the data that DAL exposes to upper level.
I think there are 2 possible points of view:
DefaultRepository class should be not in BLL but in DAL. Your code in Console should be in BLL
DefaultRepository is a BLL and EntityFramework serves as a DAL.
I any case, your entity "Student" is just a piece of data that can be shared.
If you take more specific pattern like MVC: V (View) and C (Controller) knows about M (Model).
To sum up, you can:
create a 4th project that will be shared by all projects and will contain your entities (or interfaces for these entities).
Don't bother and add link from Console to DAL. The most important that DAL doesn't have links to BLL and console.
This question already has answers here:
Ioc/DI - Why do I have to reference all layers/assemblies in application's entry point?
(4 answers)
Closed 7 years ago.
I'm developing an ASP.NET MVC application. At the same time, I'm following the book "Dependency Injection in .NET" from Mark Seemann. My solution has one Data Access layer, a Domain layer, and the Web application project. Chapter two of the book says that I should put my controllers and viewmodels in a separate Presentation layer, which is what I'm trying to do right now.
Things are going ok. My goal is to build this app with DI in mind. My web app only references the Domain layer, the Data layer only references the Domain layer. Everything is as decoupled as I can possibly make it.
However, my web application project uses Ninject as the container, and in one class called NinjectWebCommon, I have to register one of my types as follows:
kernel.Bind<ITransactionRepository>().To<TransactionRepository>();
As you can see, I now have to reference TransactionRepository which is in my Data layer. Is it ok that my web application references the Data layer? Should it? Or is there another solution to prevent this coupling? Should I move NinjectWebCommon to the Presentation layer? I'm not sure what is best practice in this case. Unfortunately the book does not go too much into detail when it comes to this Presentation layer in an ASP.NET MVC application.
Yes - this is fine. Your composition root needs a reference to everything it will work with. How else would it resolve abstractions?
There is a semantic difference between a project reference and a dependency here.
A reference is something your DI container will have to have in order to do it's job - you can't avoid that.
You avoid dependency by not writing your presentation layer code to depend upon concrete implementations of your data layer - only abstractions in your domain layer.
Put differently, a reference is a required element of the configuration, while a dependency is a choice by your implementation to depend on something.
Consider some MVC controller
public class SomeController
{
private ISomeRepository repository;
public SomeController(ISomeRepository repository)
{
this.repository = repository;
}
// ...
}
This controller has a dependency on the domain layer, which is expected. Contrast that to:
public class SomeController
{
private SomeRepository repository;
public SomeController(SomeRepository repository)
{
this.repository = repository;
}
// ...
}
This implementation has a hard dependency on the data layer, which is undesired.
So my problem is the following. My solution contains the following projects (with references):
Presentation Layer, contains Views (has a reference to Application Layer)
Application Layer, contains ViewModels (has a reference to Domain and Persistence Layer)
Domain Layer, contains all Models (no reference to anything)
Persistence Layer, stores data with Entity Framework (reference to Domain Layer)
All right, now I want to use Dependency Injection to decouple my ViewModels from Services and other stuff. Because I'm also using a dialogs, I also need to inject the IDialogService with the implementation DialogService.
Now, the DialogService uses some Presentation-specific DLLs which are only in the Presentation project so I had to implement the IDialogService interface in the Presentation project, but the Unity-Container is in the Application Layer. I think you can see what my problem is: I only have a reference from Presentation Layer to Application Layer, not the other way.
Am I doing this right and how can I solve this problem?
You are missing a layer: the Composition Root layer. This is the top-most layer of your application and it references all other layers in you application. Often you see that this layer is put into the same assembly as the presentation layer (which is fine, because layers are logical artifacts, while assemblies are physical artifacts). In the case of WPF however, it is very easy to move all WPF related stuff to a different assembly and let the start-up project consist of nothing more than the bootstrapping logic (with the container) that wires everything together.
So in general you shouldn't let each assembly be responsible of its own wiring, because that would cause a needless dependency on the container. In general only the composition root has to take a dependency on your DI library.
Also see this related question.
Background
I have a project employing n-tier architecture that exposes the business and data tiers using a service facade (WCF) to the presentation layer (ASP.NET MVC). The presentation layer is separated into concerns using separate class libraries i.e.,
Presentation Layer (ASP.NET MVC)
--- Views
--- ViewModels
--- Controllers
--- Repositories
Service Layer (WCF)
Business Layer
Data Layer
Controllers call respective repositories which in turn call the service layer.
The service layer uses data transfer objects (DTO) to facilitate the transport of business objects in a serialized format (i.e. flattened business objects) over the wire.
Problem
In the presentation layer I need to map the DTOs to their respective viewmodel objects. This works fine when I have one MVC project containing all views, viewmodels, controllers and repositories. But I want to maintain the class library separation. This introduces a problem because the service reference is added to only one of those projects (Repositories). i.e. the other projects won't recognize the DTO types that are defined in the service reference contract because the service contract won't be added.
For example, the ViewModels project won't be able to map the DTOs to ViewModels because the DTO data types won't be recognised. How would I be able to maintain this separation in the presentation layer if only the datatypes were accessible via the service contract?
the easy/best solution seems make the DTO's a separate project (a class library) and reference those from both your service layer and presentation layer.
update: that said if they always get their data having a reference to your service layer doesn't seem that bad (still not ideal of course) if they always get their data through the service layer (a.k.a they are already dependent on your service layer regardless if they have a service reference link)
In the ASP.NET MVC architecture, the Model(M) is not exactly the business layer , it seems like View Model :
Exemple
these class may be in the business Layer :
public class Student
{
}
public class Course
{
}
Sometimes we need to combine between the model class so , in the Model (in asp.net MVC application) we can find this class :
public class Education
{
List<Etudiant> etudiants = new List<Etudiant>();
List<Course> courses = new List<Course>();
}
It's a good practise to maintain the separation between the model part ( BLL layer) from the UI Layer ( asp.net MVC application for example)
I've basically kept the same architecture and kept the service reference added to the Repositories project. Instead of having a extension methods helper class that handles DTO to ViewModel mapping in the ViewModels project I have added this to the Repositories project since this is the only class that needs to be aware of DTO data types. Seemed like a decent trade off.