It seems that the HttpGet method's return type need not be an ActionResult. For example, the following method works:
[HttpGet]
[Route("list")]
public async Task<IEnumerable<MyItem>> List()
But then, how can I return a BadRequest (BadRequest("...")) in this case?
If you really have to respond with BadRequest from within the controller method, you can use the following approach as recommended by Microsoft here
[HttpGet("list")]
[ProducesResponseType(StatusCodes.Status200OK, Type = typeof(IEnumerable<MyItem>))]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async IActionResult List()
{
// Code to validate request
if(requestIsValid)
{
// Code to get IEnumerable of MyItems
return Ok(myItems);
}
else
{
return BadRequest()
}
}