Trả về kết quả

using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/products")]
public class ProductsController : ControllerBase
{
    private static readonly List<Product> Products =
        new List<Product>();

    [HttpGet("{id}")]
    public ActionResult<Product> GetById(int id)
    {
        var product =
            Products.FirstOrDefault(p => p.Id == id);
        if (product == null)
        {
            return NotFound();
        }
        return Ok(product);
    }

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

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public decimal Price { get; set; }
}

curl -i -X POST http://localhost:5000/api/products -H "Content-Type: application/json" -d @product.json
curl -i http://localhost:5000/api/products/1
curl -i http://localhost:5000/api/products/99

// SAI — không tìm thấy mà vẫn là "thành công"
using Microsoft.AspNetCore.Mvc;

public class OrdersController : ControllerBase
{
    [HttpGet("{id}")]
    public string? GetById(int id)
    {
        return null;
    }
}

// ĐÚNG
using Microsoft.AspNetCore.Mvc;

public class OrdersController : ControllerBase
{
    [HttpGet("{id}")]
    public ActionResult<string> GetById(int id)
    {
        return NotFound();
    }
}