Building Your First REST API with ASP.NET Core

Why ASP.NET Core for APIs?

ASP.NET Core has become my go-to framework for building web APIs. It's fast, cross-platform, and has excellent tooling. If you're coming from other frameworks or just starting with backend development, this guide will get you up and running.

Setting Up Your Project

First, create a new Web API project:

dotnet new webapi -n MyFirstApi
cd MyFirstApi
dotnet run

That's it! You now have a running API at https://localhost:5001. But let's make it actually do something useful.

Creating Your First Controller

Controllers handle incoming HTTP requests. Here's a simple example for managing a todo list:

[ApiController]
[Route("api/[controller]")]
public class TodosController : ControllerBase
{
    private static List<Todo> todos = new();

    [HttpGet]
    public ActionResult<IEnumerable<Todo>> GetAll()
    {
        return Ok(todos);
    }

    [HttpPost]
    public ActionResult<Todo> Create(Todo todo)
    {
        todo.Id = todos.Count + 1;
        todos.Add(todo);
        return CreatedAtAction(nameof(GetById), 
            new { id = todo.Id }, todo);
    }
}

Important Concepts

• HTTP Verbs - GET for reading, POST for creating, PUT for updating, DELETE for removing
• Status Codes - 200 OK, 201 Created, 404 Not Found, 400 Bad Request
• Routing - How URLs map to your controller actions
• Model Binding - Automatically converting request data to C# objects

Adding a Database

Most real APIs need data persistence. Entity Framework Core makes this easy:

public class ApplicationDbContext : DbContext
{
    public DbSet<Todo> Todos { get; set; }
    
    protected override void OnConfiguring(
        DbContextOptionsBuilder options)
    {
        options.UseSqlServer(connectionString);
    }
}

Best Practices I've Learned

• Always validate input data
• Use DTOs (Data Transfer Objects) instead of exposing your database models directly
• Implement proper error handling with try-catch blocks
• Add authentication and authorization from the start
• Document your API with Swagger/OpenAPI

Testing Your API

I use Postman for manual testing and xUnit for automated tests. Here's a simple test:

[Fact]
public async Task GetAll_ReturnsOkResult()
{
    var controller = new TodosController();
    var result = await controller.GetAll();
    Assert.IsType<OkObjectResult>(result.Result);
}

Next Steps

Once you're comfortable with the basics, explore:

• JWT authentication
• Rate limiting
• Caching strategies
• API versioning
• Microservices architecture

Building APIs is incredibly rewarding. You're creating the backbone that powers mobile apps, web frontends, and integrations. The skills you learn here transfer across platforms and languages.