In ASP.NET 6, the standard way to return a 404 from a Controller action is with

return NotFound();

And if you want to return a message with it, you can do

return NotFound($"Couldn't find an account with id {accountId}.");

Which returns:

Couldn't find an account with id 123.

Which isn’t ideal. Ideally we want to return a ProblemDetails like a nice API should. I found this workaround:

return Problem(statusCode: StatusCodes.Status404NotFound, detail: $"Couldn't find an account with id {accountId}");

Which returns us a ProblemDetails:

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.4",
  "title": "Not Found",
  "status": 404,
  "detail": "Couldn't find an account with id 123.",
  "traceId": "00-8292718bbb9d727dd1108abe3165deac-82f256594498617d-00"
}

Similarly, for 400s, you might think the best thing to return is

return BadRequest("Invalid accountId.");

But sadly my friend, as with NotFound(“…”) above, you’ll just get a 400 with a string in the request body. To return a ProblemDetails with a 400, you can call

ModelState.AddModelError(nameof(accountId), "must be greater than zero");
return ValidationProblem(ModelState);

Which gives you:

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "traceId": "00-3ae564a748991dee1d1a269d73e73b3f-bcdb5d6686a1b66a-00",
  "errors": {
    "accountId": [
      "must be greater than zero"
    ]
  }
}

Or if you’re not happy with the shape of that response, you could use Problem() as above:

return Problem(statusCode: StatusCodes.Status400BadRequest, detail: $"{nameof(accountId)} must be greater than zero.");

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "Bad Request",
  "status": 400,
  "detail": "accountId must be greater than zero.",
  "traceId": "00-0fc60678b8c5dee4e6d22faa40d8f12a-4e1992ddf4c43cd0-00"
}

While we’re on the topic, don’t forget to decorate your controller actions with these responses:

[SwaggerResponse(StatusCodes.Status400BadRequest, null, typeof(ProblemDetails), "application/problem+json")]
[SwaggerResponse(StatusCodes.Status404NotFound, null, typeof(ProblemDetails), "application/problem+json")]