Implementing Domain-Driven Design in ASP.NET MVC

Domain-Driven Design (DDD) is an approach to software development that focuses on complex needs by connecting the implementation to an evolving model of the core business concepts. Below, is a simplified example of implementing DDD within an ASP.NET MVC application.

Project Structure

The project is organized into several layers:

• UI Layer – The MVC project itself, which should be thin and only contain presentation logic.
• Application Layer – Contains the application logic and acts as a bridge between the UI and domain layers.
• Domain Layer – Where the business logic is implemented, including entities, value objects, domain events, and domain services.
• Infrastructure Layer – Typically contains logic for data access, file storage, etc.

Domain Layer

We start by creating our domain entities and value objects. These are the heart of our DDD approach.

public class Product
{
    public int Id { get; private set; }
    public string Name { get; private set; }
    public Money Price { get; private set; }

    public Product(string name, Money price)
    {
        Name = name;
        Price = price;
    }

    // Other domain logic
}

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

    public Money(decimal value, string currency)
    {
        Value = value;
        Currency = currency;
    }

    // Money-specific domain logic
}

Application Layer

Here’s where you define interfaces and their implementations, which will be used by the UI to communicate with the domain layer.

public interface IProductService
{
    Product CreateProduct(string name, decimal price, string currency);
}

public class ProductService : IProductService
{
    public Product CreateProduct(string name, decimal price, string currency)
    {
        var money = new Money(price, currency);
        var product = new Product(name, money);
        // Save the product to the database or perform other operations
        return product;
    }
}

Infrastructure Layer

The infrastructure layer can hold implementations for repository interfaces defined in the domain layer.

public interface IProductRepository
{
    void Add(Product product);
    // Other database operations
}

public class ProductRepository : IProductRepository
{
    public void Add(Product product)
    {
        // Implementation for adding product to the database
    }
}

UI Layer (MVC Controllers)

public class ProductController : Controller
{
    private readonly IProductService _productService;

    public ProductController(IProductService productService)
    {
        _productService = productService;
    }

    public ActionResult Create()
    {
        return View();
    }

    [HttpPost]
    public ActionResult Create(string name, decimal price, string currency)
    {
        var product = _productService.CreateProduct(name, price, currency);
        // Ideally, this should go through a DTO rather than passing the domain entity directly
        return View("Details", product);
    }
}

The above example is a very basic illustration of the layers and separation of concerns that DDD encourages. The actual implementation of DDD can get quite complex, as it aims to tackle the complexities of real-world business software.

Remember, this example is highly simplified. In a real-world application, you would need to handle things like validation, domain events, repository patterns, entity framework integrations or other ORM tools, dependency injection, and more. Always keep in mind that the main goal of DDD is to align your software structure with your business needs and model.

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