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Layered Architecture, Dependency Injection, And Dependency Inversion

There are more examples, but hopefully, you get the idea. We are hiding all the implementation details in the Infrastructure layer because it is at the top of the Onion architecture, while all of the lower layers depend on the interfaces . Even though the presenter has its own cohesive set of responsibilities, it cannot accomplish its responsibilities without leaning on its dependencies. It just so happens that its dependencies are provided to it at the time of creation. This is a type of dependency injection called constructor-based dependency injection.

The clean architecture uses the principle of dependency inversion with the strict rule that dependencies shall only exist between an outer ring to an inner ring and never the contrary. In particular, you get separation of concerns and domain logic, which makes tracking bugs, duplicates and other maintenance issues much easier. Previously, we used Microsoft’s data access stack as an example of onion-based architecture. Today, we can refer to Microsoft’s platform as an onion-based architecture that is used with both ASP.NET and Visual Studio successfully. Onion architecture is a software architectural configuration to maintain libraries and dependencies on the extremities of a software system while sustaining a strong and cohesive system core. Developing a system core which is both stable and efficient is essential when basing a system’s architecture to that of an onion.

In the following sections, we’re going to briefly talk about each high-level element of this architecture. The most common type involves division into Presentation Layer, Logic Layer / Domain Layer, and Data Layer. According to some authors, the hexagonal architecture is at the origin of the microservices architecture.

The controller only depends on interfaces, which are defined in the application core. Remember that all dependencies are toward the center. The diagram to the left depicts the Onion Architecture. This architecture is unashamedly biased toward object-oriented programming, and it puts objects before all others.

A “service” that can execute application functionality on behalf of the presenter. The presenter processes the event accordingly and may push information back to the view by means of using the View interface. Creating a SQL statement to pull the appropriate information from the database. A fan of mediocre movies and well-written documentation. He likes to play sharp guitar riffs from time to time. But it requires a certain level of complexity from the app for these benefits to truly shine.

They encapsulate the most general and high-level rules. They are the least likely to change when something external changes. For example, you would not expect these objects to be affected by a change to page navigation, or security. No operational change to any particular application should affect the entity layer. When designing the architecture of a building there are many aspects you need to consider.

Advantages of onion architecture

The term “Onion Architecture” was first coined by Jeffry Palermo back in 2008 in a series of blog posts. The architecture is intended to address the challenges faced with traditional architectures and the common problems likecouplingandseparation of concerns. Further, the biggest drawback of this architecture is unnecessary coupling that it creates. The obvious advantage of the Onion architecture is that our controller’s methods become very thin.

Infrastructure Layer

In the source code accompanying this article, you can see how I configure the Windsor container using Binsor, a great tool for configuring Windsor without XML files. Ayende Rahien developed Binsor and it allows me to configure dependencies using configurations stored outside of the application. Unfortunately, this causes the view to not just be responsible for creating its presenter, it also now has to create and be coupled to the dependencies the presenter is reliant on. That seems to defeat the purpose of going down this road in the first place. Forget about that last bullet point for a little while. I can start to remedy the first three items by introducing a layered application architecture.

The Infrastructure Layer will implement interfaces from the Application Layer to provide functionality to access external systems. These will be hooked up by the IoC container, usually in the Presentation Layer. Now i can see all my business logic concentrating on Services layer.

  • Again, another positive side-effect of this style of coding is that it allows me to easily test the behavior of this class without actually pointing it at a real database.
  • Let’s see now see what hexagonal architecture can do for you.
  • A common example of a controller could be a REST controller or any other API request handler.
  • The traditional and most commonly used web application architecture isModel-View-Controller architecture which is one of the most widely adapted and appreciated architecture throughout the industry.

Adapters are the glue between components and the outside world. They tailor the exchanges between the external world and the ports that represent the requirements of the inside of the application component. There can be several adapters for one port, for example, data can be provided here is an explanation by a user through a GUI or a command-line interface, by an automated data source, or by test scripts. Service Interface Layer– common operations like Add, Save, Delete should go in here within interfaces. You can define your transactional interfaces like (IOrderService.SubmitOrder) here.

Application #

Application ServicesLayer– the implementation of Interfaces defined in Service Interface layers comes here. The service implementation gets data from repositories and processes requests coming in from UI layer. This layer acts as a middleware to provide data from Infrastructure to UI.

This rule says that source code dependencies can only point inwards. Nothing in an inner circle can know anything at all about something in an outer circle. In particular, the name of something declared in an outer circle must not be mentioned by the code in the an inner circle. The driven actors are those that need the core application to interact with them. In this case, the application calls the external entity. Then, the driven adapter implements the port for the core to use.

The user input passed into the UI is taken by the adapter and sent to the core through the port. Both the port and the implementation of the interface will be inside the core/hexagon. All dependencies move from the user-side / user interface and server-side towards the business logic.

In the final step I register the service locator with the DependencyResolver so that all clients can access its functionality. I chose to couple the presenter to the service locator by using the static DependencyResolver class. Someone out there right now is screaming, “static classes are evil!

Understanding Onion Architecture

This approach is not new, but it is also not nearly as common as it perhaps should be. Let’s see what each of these layers represents and what should each contain. Then, we explained how we can connect all of the layers using an ASP.NET Core Web API. Great, we saw how we wired up all of the dependencies of our application. However, there are still a couple of things to take care of. We did not see how to wire up any of our dependencies.

Advantages of onion architecture

The Onion architecture was first introduced by Jeffrey Palermo, to overcome the issues of the traditional N-layered architecture approach. You now have knowledge of another arsenal of tips and techniques which you can start applying to realize more loosely coupled solutions in your application architectures. One of the main issues with the code in Listing 1 is that it takes the single responsibility principle and throws it completely out of the window. I encourage you to use the term “Onion Architecture” when speaking about architectures that adhere to the above four tenets.

Business Logic Layer

We’ve shown you how to implement the Domain layer, Service layer, and Infrastructure layer. Also, we’ve shown you the Presentation layer implementation by decoupling the controllers from the main Web application. We are using a Web API built with ASP.NET Core to create a set of RESTful API endpoints for modifying the domain entities and allowing consumers to get back the data. On the other hand, the service interfaces are public. Lazy class to ensure the lazy initialization of our services. This means that our service instances are only going to be created when we access them for the first time, and not before that.


The outermost layer is generally composed of frameworks and tools such as the Database, the Web Framework, etc. Generally you don’t write much code in this layer other than glue code that communicates to the next circle inwards. In fact your business rules simply don’t know anything at all about the outside world.


In this section of code I am coupling the presenter to SqlConnection, SqlCommand, and SqlDataReader objects. In reality, worse than the coupling is the fact that this functionality does not really belong in the presentation layer of a project. It still unnecessarily couples my presentation layer to the underlying physical database that is serving data to this application.

This can be avoided with IoC containers like Autofac with the use of Registries and assembly scanning. Application and Domain are considered the ‘core’ of the application. Application depends on Domain, but Domain depends on nothing. Onion Architecture, Hexagonal Archecture, Screaming Architecture, and others.

The Domain Layer is the heart of your application, and responsible for your core models. Models should be persistence ignorant, and encapsulate logic where possible. We want to avoid ending up with Anemic Models (i.e. models that are only collections of properties). The problem here, I think is that while the architecture doesn’t permit the top layer from talking to the domain, the sample project does exactly that, pretty much pulling the guts out.

You can look at the code that accompanies this article to get an idea of how I cleaned up the service layer class, as well as separated responsibilities into more discrete layers. The biggest offender is the coupling of UI and business onion structure logic to data access. Yes, UI is coupled to data access with this approach. Business logic can’t function if data access isn’t there. I’m intentionally ignoring infrastructure here because this typically varies from system to system.

Data Access, I/O, and Web Services are all infrastructure. Infrastructure is any code that is a commodity and does not give your application a competitive advantage. This code is most likely to change frequently as the application goes through years of maintenance. Web services are still fairly new, and the first version in .Net, ASMX, is already deprecated in favor of WCF.

This makes components exchangeable at any level and facilitates test automation. Does this mean that I am done with the current implementation of IEmployeeTask? In my opinion, the current implementation has too many responsibilities which should be placed in other objects . Rather than drill down into more abstractions, I would rather spend the remainder of this article discussing the wiring up of all of these dependencies to one another.

But things doesn’t turn out as planned and leads to very tight coupling between UI and business logic and business logic to database logic. This is because you end up writing all of your logic in server side code (mainly aspx.cs files). This scenario produces very tight coupling over the period of time and system becomes a nightmare to maintain.

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