controller404.cs

[Route("api/[controller]")]
public class CustomersController : Controller
{
    private readonly CustomerRepository _customerRepository;

    public CustomersController()
    {
        _customerRepository = new CustomerRepository();
    }

    // GET api/values/5
    [HttpGet("{id}")]
    public IActionResult Get(int id)
    {
        // Returns null if customer doesn't exist.
        var customer = _customerRepository.GetById(id);

        if (customer == null) return new StatusCodeResult(404);

        // This could throw a NullReferenceException if customer is null
        return new JsonResult(new
        {
            CustomerId = customer.CustomerId,
            Name = customer.Name
        });
    }
}