Returns nothing. If the return type is void, the web API simply delivers an empty HTTP response with the status code 204 (No Content).
Any entity type can be used as the return type for a web API action method. The problem here is that we always get the status code 200 (OK) after a successful response.
If the action returns an HttpResponseMessage, the web API automatically converts the return value into an HTTP response message, populating the response with the properties of the HttpResponseMessage object.
HttpResponseMessage
IHttpActionResult was introduced in ASP.NET Web API 2. This is an advanced HttpResponseMessage concept. IHttpActionResult ultimately returns HttpResponseMessage. It provides developers with customization and reusability features.
IHttpActionResult