Building a Clean ASP.NET Core API with C# 13, EF Core, and DDD

Overview

With the latest .NET technologies and industry best practices, we'll walk you through building a scalable, modular, and testable RESTful API. Maintainability and clean architecture principles guide the solution's design, making it ideal for enterprise development in the real world.

ASP.NET Core 8 will be used to handle HTTP requests and responses, and C# 13 (with preview features) will enable us to write cleaner and more expressive code by leveraging the latest language enhancements. We will use Entity Framework Core 8 and SQL Server to access and persist data.

The architecture adheres to Domain-Driven Design (DDD), separating concerns across layers and prioritising core domain logic. To abstract data access and ensure transactional consistency, we'll implement the Repository and Unit of Work patterns. For stability and traceability, the project also incorporates FluentValidation for input validation, as well as structured logging and centralised exception handling.

To ensure code quality and reliability, we will use xUnit for unit testing and Moq for mocking dependencies. This complete setup allows developers to build clean, testable, and extensible APIs.

Using the following methods, we will create a modular, testable, and scalable REST API:

ASP.NET Core 8
C# 13 (Preview features)
Entity Framework Core 8
MS SQL Server
Domain-Driven Design (DDD)
Repository + Unit of Work Patterns
Dependency Injection
Validation, Error Handling & Logging

Project Setup & Structure

Clean Folder Structure (Clean Architecture & DDD-Aligned)

In order to promote separation of concerns, maintainability, and testability, the project uses Clean Architecture principles and Domain-Driven Design principles. The src directory is divided into well-defined layers, each with a specific responsibility.

CompanyManagement.API: As the interface between the application layer and the outside world, CompanyManagement.API contains API controllers, dependency injection configurations, and middleware such as error handling and logging.
CompanyManagement.Application: In CompanyManagement.Application, the business logic is encapsulated in services (use cases), data transfer objects (DTOs), and command/query handlers. Without concern for persistence or infrastructure, this layer coordinates tasks and enforces application rules.
CompanyManagement.Domain: CompanyManagement.Domain defines the business model through entities, interfaces, enums, and value objects. This layer is completely independent of any other project and represents the domain logic.
CompanyManagement.Infrastructure: The CompanyManagement.Infrastructure class implements the technical details necessary to support the application and domain layers. This includes Entity Framework Core configurations, the DbContext, repository implementations, and database migrations.
CompanyManagement.Tests: CompanyManagement.Tests contains unit and integration tests to help maintain code quality and prevent regressions by testing each component of the system in isolation or as part of a broader workflow.

As a result of this layered structure, the application is able to evolve and scale while keeping the codebase clean, decoupled, and easy to test.

Step-by-Step Implementation

Define the Domain Model

Company.cs – Domain Entity

In Domain-Driven Design (DDD), the domain model captures the core business logic of your application. In the CompanyManagement.Domain.Entities namespace, the Company entity encapsulates key business rules and states of a real-world company.

There are three primary properties in the Company class:

Id: At the time of creation, a unique identifier (Guid) is generated.
Name: A company's name, which is immutable from outside the class, is its name.
EstablishedOn: The company's founding date is marked by the EstablishedOn property.

As Entity Framework Core (EF Core) requires a parameterless constructor for materialisation, the constructor is intentionally private. To ensure that Id is always generated and that required fields (Name, EstablishedOn) are always initialised during creation, a public constructor is provided.

This method contains a guard clause to ensure the new name is not null, empty, or whitespace, enforcing business rules directly within the domain model.

Rich domain modelling in DDD adheres to encapsulation, immutability (where appropriate), and self-validation, which are key principles.

Company.cs

 namespace CompanyManagement.Domain.Entities;

public class Company
{
    public Guid Id { get; private set; }
    public string Name { get; private set; } = string.Empty;  
    public DateTime EstablishedOn { get; private set; }

    private Company() { }  

    public Company(string name, DateTime establishedOn)
    {
        Id = Guid.NewGuid();
        Name = name;
        EstablishedOn = establishedOn;
    }

    public void Rename(string newName)
    {
        if (string.IsNullOrWhiteSpace(newName))
            throw new ArgumentException("Name cannot be empty");

        Name = newname;
    }
}

Define the Repository Interface

In Domain-Driven Design (DDD), the repository pattern provides an abstraction over data persistence, enabling the domain layer to remain independent from infrastructure concerns like databases or external APIs. The ICompanyRepository interface, defined in the CompanyManagement.Domain.Interfaces namespace, outlines a contract for working with Company aggregates.

This interface declares the fundamental CRUD operations required to interact with the Company entity:

Task<Company?> GetByIdAsync(Guid id): Asynchronously retrieves a company by its unique identifier. Returns null if no match is found.
GetAllAsync(): Returns a list of all the companies in the system.
Task AddAsync(Company company): Asynchronously adds a new Company to the data store using the AddAsync(Company company) task.
void Update(Company company): Updates an existing company. This is typically used when domain methods like Rename modify the entity's status.
void Delete(Company company): Removes a company from the database.

The rest of the application depends only on abstractions, not implementations, when this interface is defined in the domain layer. A key component of Clean Architecture, the Dependency Inversion Principle (DIP) promotes loose coupling, testability (e.g., mocks in unit tests), and is based on loose coupling.

ICompanyRepository.cs

using CompanyManagement.Domain.Entities;

namespace CompanyManagement.Domain.Interfaces;

public interface ICompanyRepository
{
    Task<Company?> GetByIdAsync(Guid id);
    Task<List<Company>> GetAllAsync();
    Task AddAsync(Company company);
    void Update(Company company);
    void Delete(Company company);
}

EF Core Implementation

As the heart of the Entity Framework Core data access layer, AppDbContext represents a session with the SQL Server database, allowing us to query and save domain entity instances.

Input into the constructor is handled through dependency injection, which allows the application to configure the context externally, such as setting the connection string or enabling SQL Server-specific functionality.

The Companies property is a strongly typed DbSet<Company>, which EF Core uses to track and manage Company entities in the database. It abstracts the Companies table and allows you to perform operations like querying, inserting, updating, and deleting data through LINQ and asynchronous methods.

The OnModelCreating method is overridden to configure entity mappings using the Fluent API. Here, we map the Company entity to the Companies table explicitly using modelBuilder.Entity<Company>().ToTable("Companies"). This approach provides flexibility for configuring additional constraints, relationships, and database-specific settings in the future — while keeping the domain model clean and free of persistence concerns.

We follow the Separation of Concerns principle by isolating all database configurations within AppDbContext, ensuring our domain remains pure and focused only on business logic.

AppDbContext.cs

using CompanyManagement.Domain.Entities;
using Microsoft.EntityFrameworkCore;

namespace CompanyManagement.Infrastructure.Data;

public class AppDbContext : DbContext 
{
    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options) { }

    public DbSet<Company> Companies => Set<Company>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Company>().ToTable("Companies");
    }
}

For managing Company entities in the database, the CompanyRepository class implements the ICompanyRepository interface concretely. The application leverages Entity Framework Core and is injected with an AppDbContext instance, so that it can interact directly with the database context.

By using ToListAsync(), the GetAllAsync method retrieves all Company records from the database asynchronously. The GetByIdAsync method locates a specific company using its unique identifier (Guid id), returning null otherwise.

With AddAsync, a new Company entity is asynchronously added to the context's change tracker, preparing it for insertion into the database when SaveChangesAsync() is called. Update marks an existing company entity as modified, and Delete flags an entity for deletion.

Using the Repository Pattern, the data access logic is abstracted from the rest of the application, which keeps the underlying persistence mechanism hidden. As a result, we can swap or enhance persistence strategies without affecting the domain or application layers, promoting separation of concerns, testability, and maintainability.

CompanyRepository.cs

using CompanyManagement.Domain.Entities;
using CompanyManagement.Domain.Interfaces;
using CompanyManagement.Infrastructure.Data;
using Microsoft.EntityFrameworkCore;

namespace CompanyManagement.Infrastructure.Repositories;

public class CompanyRepository(AppDbContext context) : ICompanyRepository
{
    public Task<List<Company>> GetAllAsync() => context.Companies.ToListAsync();
    public Task<Company?> GetByIdAsync(Guid id) => context.Companies.FindAsync(id).AsTask();
    public Task AddAsync(Company company) => context.Companies.AddAsync(company).AsTask();
    public void Update(Company company) => context.Companies.Update(company);
    public void Delete(Company company) => context.Companies.Remove(company);
}

Application Layer – Services & DTOs

As part of the Application Layer, Data Transfer Objects (DTOs) are crucial to separating external inputs from internal domain models. With C# 9+, the CreateCompanyDTO is an immutable and simple data structure that encapsulates only the necessary data for creating a new company, such as its name and its establishment date.

Using a DTO like CreateCompanyDto ensures that the API or service layer receives only the necessary information while maintaining a clear boundary from domain entities. In addition to improving maintainability, validation, and security, it simplifies testing and supports serialization/deserialization out of the box, which eliminates over-posting.

It is a clean, minimal contract aligned with Clean Architecture and Single Responsibility Principle by protecting the domain from direct external access.

CreateCompanyDto.cs

 namespace CompanyManagement.Application.DTOs;

public record CreateCompanyDto(string Name, DateTime EstablishedOn);

In this class, business operations related to Company entities are handled by the core application service. When persisting changes, it relies on two key abstractions: ICompanyRepository for data access and IUnitOfWork for transactional consistency management.

In the CreateAsync method, a new company is created by instantiating a new Company domain entity based on the CreateCompanyDto provided, then delegating to the repository and adding it asynchronously. Then it commits the transaction to the database by calling the SaveChangesAsync method on the unit of work. Finally, the caller receives the unique identifier (Id) for the newly created company that can be referenced in the future.

Asynchronously retrieves all existing companies by invoking the corresponding repository method, returning a list of Company entities.

The service contains business logic and coordinates domain operations, while the repository abstracts persistence details. By using interfaces to support easy unit testing, it follows best practices for asynchronous programming, dependency injection, and dependency injection.

CompanyService.cs

using CompanyManagement.Application.DTOs;
using CompanyManagement.Domain.Entities;
using CompanyManagement.Domain.Interfaces;

namespace CompanyManagement.Application.Services;

public class CompanyService(ICompanyRepository repository, IUnitOfWork unitOfWork)
{
    public async Task<Guid> CreateAsync(CreateCompanyDto dto)
    {
        var company = new Company(dto.Name, dto.EstablishedOn);
        await repository.AddAsync(company);
        await unitOfWork.SaveChangesAsync();
        return company.Id;
    }

    public async Task<List<Company>> GetAllAsync() => await repository.GetAllAsync();
}

Unit of Work

IUnitOfWork represents a fundamental pattern for managing data persistence and transactional consistency in an application. It abstracts the concept of a "unit of work," which encapsulates a series of operations that should be treated as a single atomic transaction.

The SaveChangesAsync() method commits all pending changes to the underlying data store asynchronously. This method returns an integer indicating the number of state entries committed.

By relying on this abstraction, the application ensures that all modifications across multiple repositories can be coordinated and saved together, preserving data integrity and consistency. Furthermore, implementations can be mocked or swapped without changing the consuming code, improving testability and separation of concerns.

IUnitOfWork.cs

namespace CompanyManagement.Domain.Interfaces;

public interface IUnitOfWork
{
    Task<int> SaveChangesAsync();
}

The UnitOfWork class implements the IUnitOfWork interface, encapsulating the transaction management logic for the application. It depends on the AppDbContext, which represents the Entity Framework Core database context.

It simply delegated the call to context.SaveChangesAsync(), asynchronously persisting all tracked changes. This ensures that any changes made through repositories within a unit of work are saved as one atomic operation.

Using the UnitOfWork class to centralize the commit logic makes it easier to coordinate multiple repository operations under one transaction, while also supporting dependency injection and testing.

UnitOfWork.cs

using CompanyManagement.Domain.Interfaces;

namespace CompanyManagement.Infrastructure.Data;

public class UnitOfWork(AppDbContext context) : IUnitOfWork
{
    public Task<int> SaveChangesAsync() => context.SaveChangesAsync();
}

API Layer

ASP.NET Core's Dependency Injection (DI) container is used to wire up essential services in Program.cs.

In order to register the AppDbContext with the DI container, AddDbContext is first used. The connection string is retrieved from the application's configuration under the key "Default" and configures Entity Framework Core to use SQL Server as the database provider. As a result, the application can access the database seamlessly.

Through AddScoped, the repository and unit of work abstractions are then mapped to their concrete implementations. The result is that a new instance of CompanyRepository and UnitOfWork is created and shared for every HTTP request, providing scoped lifetime management for database operations.

Also registered as a scoped service is the CompanyService, which contains the business logic for managing companies. With this setup, controllers and other components receive these dependencies via constructor injection, thereby promoting loose coupling, testability, and separation of concerns in the API layer.

ServiceRegistration.cs

using CompanyManagement.Application.Services;
using CompanyManagement.Domain.Interfaces;
using CompanyManagement.Infrastructure.Data;
using CompanyManagement.Infrastructure.Repositories;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

namespace CompanyManagement.Infrastructure.DependencyInjection;

public static class ServiceRegistration
{
    public static IServiceCollection AddInfrastructure(this IServiceCollection services, IConfiguration configuration)
    {
        services.AddDbContext<AppDbContext>(options =>
            options.UseSqlServer(configuration.GetConnectionString("Default")));

        services.AddScoped<ICompanyRepository, CompanyRepository>();
        services.AddScoped<IUnitOfWork, UnitOfWork>();
        services.AddScoped<CompanyService>();

        return services;
    }
}

SwaggerConfiguration.cs

using Microsoft.OpenApi.Models;

namespace CompanyManagement.API.Configurations;

public static class SwaggerConfiguration
{
    public static IServiceCollection AddSwaggerDocumentation(this IServiceCollection services)
    {
        services.AddEndpointsApiExplorer();

        services.AddSwaggerGen(options =>
        {
            options.SwaggerDoc("v1", new OpenApiInfo
            {
                Title = "Company API",
                Version = "v1",
                Description = "API for managing companies using Clean Architecture"
            });

            // Optional: Include XML comments
            var xmlFilename = $"{System.Reflection.Assembly.GetExecutingAssembly().GetName().Name}.xml";
            var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFilename);
            if (File.Exists(xmlPath))
            {
                options.IncludeXmlComments(xmlPath);
            }
        });

        return services;
    }
}

SwaggerMiddleware.cs

namespace CompanyManagement.API.Middleware;

public static class SwaggerMiddleware
{
    public static IApplicationBuilder UseSwaggerDocumentation(this IApplicationBuilder app)
    {
        app.UseSwagger();

        app.UseSwaggerUI(options =>
        {
            options.SwaggerEndpoint("/swagger/v1/swagger.json", "Company API V1");
        });

        return app;
    }
}

Program.cs

using CompanyManagement.API.Configurations;
using CompanyManagement.API.Endpoints;
using CompanyManagement.API.Middleware;
using CompanyManagement.Infrastructure.DependencyInjection;
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddOpenApi();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerDocumentation();

var app = builder.Build();

app.UseSwagger();
app.UseSwaggerDocumentation();
builder.Services.AddInfrastructure(builder.Configuration);

 .  
if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
}
app.MapCompanyEndpoints();

app.UseHttpsRedirection();

appsettings.json

{
  "ConnectionStrings": {
    "Default": "Server=localhost;Database=CompanyDb;Trusted_Connection=True;MultipleActiveResultSets=true"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

The CompanyEndpoints is the REST API entry point that handles HTTP requests related to company management. Decorated with the [ApiController] attribute, it features automatic model validation, binding, and API-specific conventions. The controller routes are prefixed with "api/[controller]", which means they will be based on the controller name (api/company).

Using constructor injection, the controller receives an instance of CompanyService, which enables it to delegate business logic operations to the application layer.

In the Create action, a CreateCompanyDto object is accepted from the request body, the service's CreateAsync method is called, and a 201 Created response is returned with the new resource's location header.
When an HTTP GET request is made with a company ID in the route, GetById retrieves all companies via the service, searches for companies with the matching ID, and returns 200 OK if a match is found, or 404 Not Found otherwise.
A 200 OK response is returned by the GetAll action when HTTP GET requests are made to retrieve all companies.

By offloading business logic to the service layer, this controller adheres to the Single Responsibility Principle and ensures maintainability and testability.

CompanyEndpoints.cs

using CompanyManagement.Application.DTOs;
using CompanyManagement.Application.Services;

namespace CompanyManagement.API.Endpoints;

public static class CompanyEndpoints
{
    public static void MapCompanyEndpoints(this WebApplication app)
    {
        app.MapPost("/api/companies", async (CreateCompanyDto dto, CompanyService service) =>
        {
            var id = await service.CreateAsync(dto);
            return Results.Created($"/api/companies/{id}", new { id });
        });

        app.MapGet("/api/companies", async (CompanyService service) =>
        {
            var companies = await service.GetAllAsync();
            return Results.Ok(companies);
        });

        app.MapGet("/api/companies/{id:guid}", async (Guid id, CompanyService service) =>
        {
            var companies = await service.GetAllAsync();
            var match = companies.FirstOrDefault(c => c.Id == id);
            return match is not null ? Results.Ok(match) : Results.NotFound();
        });
    }
}

Best Practices

Several key best practices are followed in this project to ensure that the code is clean, maintainable, and scalable. For instance, Data Transfer Objects (DTOs) are used to avoid exposing domain entities directly to external clients, improving security and abstraction. The Single Responsibility Principle (SRP) is applied by ensuring each layer has a distinct, focused responsibility, resulting in improved code organisation and clarity.

The project uses Dependency Injection to promote flexibility, ease testing, and maintain separation of concerns. For I/O-bound operations such as database access, asynchronous programming with async/await improves application responsiveness and scalability.

As a result of exception handling middleware, error logging is consistent, and maintenance is simplified. Finally, the project includes comprehensive unit testing using xUnit and Moq frameworks for each layer, which ensures code quality and reliability in the future.

Best Practice	Explanation
Use DTOs	Avoid exposing domain entities to the outside
SRP	Each layer does only one thing well
Dependency Injection	For flexibility, testing, and separation
Async/Await	For I/O operations like DB access
Exception Handling Middleware	Centralised error logging
Unit Testing	Write xUnit + Moq-based tests per layer

xUnit Test Example

By using the xUnit framework and Moq for mocking dependencies, the CompanyServiceTests class demonstrates how to run unit tests on the CompanyService. To isolate the service logic from external dependencies, mock implementations of the ICompanyRepository and IUnitOfWork interfaces are created in the test method CreateAsync_ShouldReturnNewId.

A sample CreateCompanyDto object representing a new company is created in the test by instantiating the CompanyService with these mocks. A new company ID is generated successfully by calling the CreateAsync method of the service and verifying that the GUID returned is not empty. A unit test ensures that business logic is correct regardless of database or infrastructure issues.

CompanyServiceTests.cs

using CompanyManagement.Application.DTOs;
using CompanyManagement.Application.Services;
using CompanyManagement.Domain.Interfaces;
using Moq;

namespace CompanyManagement.Tests.Unit.Services;

public class CompanyServiceTests
{
    [Fact]
    public async Task CreateAsync_ShouldReturnNewId()
    {
        // Arrange
        var repoMock = new Mock<ICompanyRepository>();
        var uowMock = new Mock<IUnitOfWork>();

        var service = new CompanyService(repoMock.Object, uowMock.Object);
        var dto = new CreateCompanyDto("Ziggy Rafiq Test Co", DateTime.Now);

        // Act
        var result = await service.CreateAsync(dto);

        // Assert
        Assert.NotEqual(Guid.Empty, result);
    }
}

Sample Folder Structure

At the root of the project, the CompanyManagement folder contains five main subfolders aligned with Clean Architecture principles.

The CompanyManagement.API file contains the API entry point as well as the Controllers folder for web controllers and Program.cs for application configuration.
The CompanyManagement.Application contains the business logic layer and contains folders for DTOs (Data Transfer Objects) and Services.
CompanyManagement.Domain defines the core domain with folders for Entities, Domain Interfaces, and Enums.
Implementation details such as database access and persistence are organized in Data for database contexts, Repositories for data access classes, and Migrations for managing schema changes in CompanyManagement.Infrastructure.
A Unit folder contains unit tests to verify the functionality of the different layers in CompanyManagement.Tests.

Separation of concerns, maintainability, and scalability are all promoted by this structure.

Summary

By leveraging several modern techniques and best practices, we have shown how to architect an ASP.NET Core API that is clean and maintainable. To build a scalable and loosely coupled architecture, we used C# 13 to write expressive and up-to-date code while applying Domain-Driven Design (DDD) principles. To ensure robust and reliable storage, we integrated Microsoft SQL Server and Entity Framework Core for data persistence. In order to keep concerns separated and the codebase organised, the design follows solid layering concepts and implements well-known patterns, including Repository, Unit of Work, and Data Transfer Objects (DTOs). Lastly, we stressed the importance of quality assurance by implementing unit testing with xUnit and Moq, which ensured that each layer of the application was thoroughly tested. The source code for this article can be found on my GitHub Repository Ziggy Rafiq.