.NET APIs with Domain-Driven Design

Introduction:

I often come across design patterns that can greatly enhance the quality and maintainability of software applications. One such design pattern that has gained popularity in recent years is Domain-Driven Design (DDD). DDD is an approach that focuses on aligning software design with the business domain, making the codebase more robust, maintainable, and adaptable to changing business requirements. In this blog, I will illustrate how DDD can be practically applied in a .NET API using a fun business case example with PetMe, a pet adoption platform.

Step 1: Understand the Business Domain

The first step in applying DDD is to thoroughly understand the business domain. This involves collaborating with domain experts, such as business analysts or product owners, to gain deep insights into the business requirements, processes, and terminologies. In our PetMe API example, we need to understand the pet adoption process, including the entities involved (e.g., listings, pets, users), their relationships, and the business rules that govern them.

For example, in PetMe, a user can create a listing to put a pet up for adoption. The listing has attributes like pet name, age, breed, and price. Users can also inquire about listings, make offers, and process transactions to finalize the adoption. Understanding these business requirements and processes is crucial for building a domain model that accurately reflects the business domain.

Step 2: Create a Rich Domain Model

Once we have a deep understanding of the business domain, we can create a rich domain model that captures the essence of the domain. The domain model is the heart of DDD and consists of entities, value objects, and domain events.

Entities represent objects with unique identities that have a lifecycle and are mutable. In our PetMe API, we can create an entity class called “Listing” that represents a pet adoption listing, and another entity class called “User” that represents a user who interacts with the listings. Here’s an example implementation in C# using ASP.NET Core:

public class Listing
{
    public Guid Id { get; private set; }
    public string PetName { get; private set; }
    public int Age { get; private set; }
    public string Breed { get; private set; }
    public Price Price { get; private set; }

    // Constructor
    public Listing(string petName, int age, string breed, Price…