IHttpActionResult is now effectively IActionResult, and to return an Ok with a return object, you’d use return new ObjectResult(...);
So effectively something like this:
public IActionResult Get(int id)
{
if (id == 1) return HttpNotFound("not found!");
return new ObjectResult("value: " + id);
}
Here’s a good article with more detail:
http://www.asp.net/vnext/overview/aspnet-vnext/create-a-web-api-with-mvc-6