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Controllers and how we define it in ASP.NET Core WebAPI

What Are Controllers ?

Controllers are responsible for handling HTTP requests and returning responses. They act as an intermediary between the client (frontend or API consumer) and the business logic (services, database, etc.).

Why Do We Use Controllers?

  • Handle HTTP Requests → They process GET, POST, PUT, DELETE requests.
  • Encapsulate Business Logic → Keep the code structured and organized.
  • Return Responses → Controllers send responses in JSON, XML, or other formats.
  • Routing → They define endpoints and determine which function runs for each request.

How we define controllers

Open-up project in VSCODE and goto backend directory and create Controllers directory. Inside this directory we will be creating our controllers for Todo and User models. Let's explain for Todo.

Step-1

Create a class called TodoController.cs and write these into the class.

using Microsoft.AspNetCore.Mvc;
using backend.Data;

namespace backend.Controllers
{
    [ApiController]
    [Route("api/todo")]
    public class TodoController : ControllerBase
    {
        private readonly ApplicationDbContext _context;
        public TodoController(ApplicationDbContext context)
        {
            _context = context;
        }

        [HttpGet]
        public IActionResult Get()
        {
            var todos = _context.Todos.ToList();
            return Ok(todos);
        }

        [HttpGet("{id}")]
        public IActionResult GetById([FromRoute] Guid id)
        {
            var todo = _context.Todos.Find(id);
            if (todo == null)
            {
                return NotFound();
            }
            return Ok(todo);
        }
    }
}

Explaination of this code block

Controller Basics

using Microsoft.AspNetCore.Mvc;
using backend.Data;
  • using Microsoft.AspNetCore.Mvc; → Imports ASP.NET Core MVC framework to handle HTTP requests.
  • using backend.Data; → Imports the ApplicationDbContext class, which manages database access.

Defining the Controller

[ApiController]
[Route("api/todo")]
public class TodoController : ControllerBase
  • [ApiController] → Marks this class as an API controller (automatic model validation, error handling, etc.).
  • [Route("api/todo")] → Defines the base URL for this controller as /api/todo.
  • ControllerBase → A lightweight controller that does not use Views (only for APIs).

Dependency Injection (Constructor)

private readonly ApplicationDbContext _context;
public TodoController(ApplicationDbContext context)
{
    _context = context;
}
  • ApplicationDbContext _context; → Stores the database context.
  • TodoController(ApplicationDbContext context) → Injects the database context so we can interact with the database.

GET Endpoint - Fetch All To-Do Items

[HttpGet]
public IActionResult Get()
{
    var todos = _context.Todos.ToList();
    return Ok(todos);
}
  • [HttpGet] → Handles HTTP GET requests to /api/todo.
  • _context.Todos.ToList(); → Fetches all Todo items from the database.
  • return Ok(todos); → Returns 200 OK with the list of todos.

GET Endpoint - Fetch a Single To-Do by ID

[HttpGet("{id}")]
public IActionResult GetById([FromRoute] Guid id)
{
    var todo = _context.Todos.Find(id);
    if (todo == null)
    {
        return NotFound();
    }
    return Ok(todo);
}
  • [HttpGet("{id}")] → Handles GET requests like /api/todo/{id}.
  • [FromRoute] Guid id → Extracts the id from the URL.
  • _context.Todos.Find(id); → Searches for the Todo item in the database.
  • return NotFound(); → Returns 404 Not Found if no item is found.
  • return Ok(todo); → Returns 200 OK with the todo item.

Step-2

To make the controller workable we need to add something to Program.cs.

builder.Services.AddControllers()

  • Registers the MVC framework in the Dependency Injection (DI) container.
  • Enables controller-based routing for handling HTTP requests.
  • Must be called before builder.Build().

app.MapControllers()

  • Maps controller endpoints to the app’s request pipeline.
  • Ensures HTTP requests are routed to the correct controller actions.
  • Must be called before app.Run().

Consequences of Missing These

  • Without AddControllers(), controllers won’t be recognized.
  • Without MapControllers(), routes won’t be registered, leading to 404 Not Found errors.

Why Ordering Matters

  • builder.Build() => Finalizes service registrations and builds the app.
  • app.Run() => Starts the app; no further middleware or endpoint mappings can be added.