Why should I return ActionResult instead of object?

Question

Assuming that I have application .NET Core 2.1 Web API + Angular 7

Why should I always returns ActionResult? Is there any difference between this:

public ActionResult<MyDTO> GetData(){
    return new MyDTO();
}

and that:

public MyDTO GetData(){
    return new MyDTO();
}

In both cases I'll receive object on UI, and both returns code 200. So is just good practice to using ActionResult, or what?

Answer 1

When you use this:

public MyDTO GetData(){
    return new MyDTO();
}

You cannot return anything that's not an instance of MyDTO, besides throwing an exception.

When you use IActionResult<T> you are saying that you may return an instance of MyDTO, but you don't have to. You can return BadRequest, InternalServerError or whatever suits the API/business needs.

For example:

public IActionResult<MyDTO> GetData() 
{
    if (!User.Identity.IsAuthenticated)
    {
        return Forbidden();
    }

    var data = _someService.GetSomeData();

    if (data == null)
    {
        return NotFound();
    }

    return Ok(data);
}