using System.ComponentModel.DataAnnotations;
using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("things")]
public class ThingsController : ControllerBase
{
    [HttpPost("number-guess")]
    [ProducesResponseType(200)]
    public ActionResult<string> GuessNumber([Required] int number)
    {
        if (number != 4)
        {
            return BadRequest("that was not my number");
//          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Action method returns undeclared status code '400'.
        }

        return Ok("you guessed my number");
    }
}

return ValidationProblem(title: "that was not my number";

return Problem(title: "that was not my number", statusCode: 400);