ASP.NET Core REST APIs

After this lesson, you will be able to: Build a REST API in ASP.NET Core with controllers or minimal APIs, routing, middleware, and dependency injection.

ASP.NET Core is the .NET web framework. Fastest mainstream web framework (per TechEmpower benchmarks). Built-in DI, middleware pipeline, OpenAPI support. The bread and butter of C# backend jobs.

Prerequisites:async/await in C#

Minimal API (recommended for new projects)

Lightweight; great for microservices.

tsx

// Create: dotnet new webapi -minimal -o myapi && cd myapi && dotnet run

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSingleton<IUserStore, InMemoryUserStore>();

var app = builder.Build();

// Routes
app.MapGet("/", () => "Hello!");

app.MapGet("/users/{id:int}", async (int id, IUserStore store) => {
    var user = await store.FindByIdAsync(id);
    return user is null ? Results.NotFound() : Results.Ok(user);
});

app.MapPost("/users", async (User user, IUserStore store) => {
    await store.SaveAsync(user);
    return Results.Created($"/users/{user.Id}", user);
});

app.Run();

// User type
public record User(int Id, string Name, string Email);

Controller-based API (classic)

Preferred for big APIs with many routes.

tsx

// Controllers/UsersController.cs
[ApiController]
[Route("users")]
public class UsersController : ControllerBase {
    private readonly IUserStore _store;

    public UsersController(IUserStore store) {
        _store = store;
    }

    [HttpGet("{id:int}")]
    public async Task<ActionResult<User>> GetById(int id) {
        var user = await _store.FindByIdAsync(id);
        return user is null ? NotFound() : Ok(user);
    }

    [HttpPost]
    public async Task<ActionResult<User>> Create(User user) {
        await _store.SaveAsync(user);
        return CreatedAtAction(nameof(GetById), new { id = user.Id }, user);
    }
}

// Program.cs
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllers();
builder.Services.AddSingleton<IUserStore, InMemoryUserStore>();
var app = builder.Build();
app.MapControllers();
app.Run();

Middleware pipeline

Request-response chain. Order matters.

tsx

var app = builder.Build();

// Exception handler, outermost
app.UseExceptionHandler("/error");

// HTTPS redirect
app.UseHttpsRedirection();

// CORS
app.UseCors("AllowAll");

// Auth
app.UseAuthentication();
app.UseAuthorization();

// Custom middleware (inline)
app.Use(async (context, next) => {
    Console.WriteLine($"{context.Request.Method} {context.Request.Path}");
    await next.Invoke();
    Console.WriteLine($"  → {context.Response.StatusCode}");
});

// Routes, innermost
app.MapControllers();

app.Run();

Common mistakes

Wrong DI lifetime (Scoped vs Singleton vs Transient, read carefully). Skipping `AddCors` then puzzling over CORS errors. Forgetting `app.MapControllers()`, routes won't match. Putting business logic in controllers (extract to services).

←Async/Await in C#

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