An HTTP response includes more than just HTML — it also includes an HTTP status code. A popular one is 404, which means that a requested resource was not found. For example, a user requests a profile page of a non-existent person.

We can generate that kind of response using NotFound().

In this example, a request with the username "Machiavelli" will lead to a 404 response:

public IActionResult OnPost(string username)
{
  if (username == "Machiavelli")
  {
    return NotFound();
  }

  return Page();
}