Implementing Domain-Driven Design (DDD) in .NET Core

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June 27, 2020

As software applications grow in size and complexity, maintaining clear boundaries and organizing code effectively becomes increasingly important. Domain-Driven Design (DDD) provides a methodology to tackle these challenges by focusing on the core domain and its logic.

In this post, we’ll explore how to implement Domain-Driven Design principles in a .NET Core application. We’ll cover fundamental DDD concepts such as Entities, Value Objects, and Aggregates, and how to apply them to structure a large-scale .NET Core application for maintainability, flexibility, and scalability.

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1. What is Domain-Driven Design (DDD)?

Domain-Driven Design (DDD) is a set of principles and practices for building complex software applications that are closely aligned with the business domain they serve. DDD focuses on the following aspects:

• Understanding the business domain: Collaborating with domain experts to understand the problem space.
• Modeling the domain: Creating models that represent the business concepts and logic.
• Ubiquitous Language: Ensuring everyone involved in the project (developers, domain experts, etc.) uses the same vocabulary to describe the domain.

In a .NET Core application, DDD can help you create a well-organized codebase by separating the core business logic from other parts of the application, such as infrastructure and user interfaces.

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2. Key DDD Concepts in .NET Core

Now, let’s explore the key concepts of DDD and how they map to .NET Core:

a. Entities

An Entity is an object that has a unique identity and can change over time. Entities are typically used to represent business objects that have lifecycle states.

For example, consider a Customer entity in a business domain. Each customer will have a unique identifier (ID), and the customer’s state (e.g., name, address, etc.) can change over time.

• Example of an Entity in .NET Core:

  public class Customer
  {
      public int Id { get; private set; }
      public string Name { get; private set; }
      public string Address { get; private set; }

      public Customer(int id, string name, string address)
      {
          Id = id;
          Name = name;
          Address = address;
      }

      public void UpdateAddress(string newAddress)
      {
          Address = newAddress;
      }
  }

In the above code, the Customer entity has an ID and can update its address over time.

b. Value Objects

Value Objects are objects that do not have a unique identity and are defined by their attributes. These are used to represent concepts that are immutable and don’t change once created. A Value Object is often used for things like money, measurements, or dates.

For example, you might create a Money value object to represent amounts with a currency.

• Example of a Value Object in .NET Core:

  public class Money
  {
      public decimal Amount { get; private set; }
      public string Currency { get; private set; }

      public Money(decimal amount, string currency)
      {
          Amount = amount;
          Currency = currency;
      }

      public Money Add(Money other)
      {
          if (Currency != other.Currency)
              throw new InvalidOperationException("Cannot add money with different currencies.");

          return new Money(Amount + other.Amount, Currency);
      }
  }

In this example, the Money value object is immutable. Once it’s created with an amount and a currency, its values cannot change.

c. Aggregates

An Aggregate is a group of related entities and value objects that are treated as a single unit for data changes. Aggregates ensure the consistency of the business rules within the boundary. Aggregates have a root entity called the Aggregate Root, which is the only entity that can be accessed directly from outside the aggregate.

For example, in an Order aggregate, you might have an Order as the root, and OrderItems as part of the aggregate. You interact with the aggregate root, and all modifications to the aggregate (such as adding or removing order items) are done through the root.

• Example of an Aggregate in .NET Core:

  public class Order
  {
      public int Id { get; private set; }
      public List<OrderItem> Items { get; private set; }

      public Order(int id)
      {
          Id = id;
          Items = new List<OrderItem>();
      }

      public void AddItem(OrderItem item)
      {
          Items.Add(item);
      }
  }

  public class OrderItem
  {
      public int Id { get; private set; }
      public string ProductName { get; private set; }

      public OrderItem(int id, string productName)
      {
          Id = id;
          ProductName = productName;
      }
  }

In this case, the Order is the aggregate root, and the OrderItems are part of the Order aggregate. You would interact with the Order entity to manage the items within it.

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3. Structuring a Large-Scale .NET Core Application Using DDD

When structuring a large-scale .NET Core application using DDD, it’s essential to organize your code into layers and boundaries to reflect the business logic effectively. A common approach to structuring DDD applications in .NET Core includes the following layers:

• Domain Layer: Contains all your business logic, including entities, value objects, aggregates, and domain services.
• Application Layer: Contains use cases and orchestrates operations on the domain model.
• Infrastructure Layer: Deals with data persistence, external APIs, and system integrations.
• Presentation Layer: Provides the user interface (e.g., API controllers, web pages, etc.).

The Domain Layer is the heart of your application, and it should be independent of other layers, such as the infrastructure or presentation layers.

a. Example Directory Structure in .NET Core:

src/
|-- Domain/
|   |-- Entities/
|   |-- ValueObjects/
|   |-- Aggregates/
|   |-- Services/
|-- Application/
|   |-- Commands/
|   |-- Queries/
|   |-- Handlers/
|-- Infrastructure/
|   |-- Data/
|   |-- Repositories/
|-- WebApi/
|   |-- Controllers/
|   |-- Models/

In this structure:

• The Domain layer contains entities, value objects, and aggregates that define your business logic.
• The Application layer contains commands, queries, and handlers that orchestrate the interactions between your domain objects.
• The Infrastructure layer provides data access and implements repository patterns for persistence.
• The WebApi layer is where you would expose your application via RESTful APIs to the user interface.

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4. Conclusion

Implementing Domain-Driven Design (DDD) in .NET Core can significantly improve the maintainability, scalability, and flexibility of your application. By focusing on the business domain, organizing your application into well-defined layers, and adhering to DDD principles, you can ensure that your application evolves with the business requirements while keeping the codebase manageable.

In this post, we’ve covered the essential concepts of Entities, Value Objects, and Aggregates, and how to apply them in a .NET Core application. We’ve also explored how to structure large-scale applications using DDD principles to ensure your app is well-organized, modular, and maintainable.

Stay tuned for more posts on DDD, and happy coding!

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