Get the current user, within an ApiController action, without passing the userID as a parameter

How do we get the current user, within an secure ApiController action, without passing the userName or userId as a parameter?

We assume that this is available, because we are within a secure action. Being in a secure action means that the user has already authenticated and the request has her bearer token. Given that WebApi has authorized the user, there may be a built in way to access the userId, without having to pass it as an action parameter.

In WebApi 2 you can use RequestContext.Principal from within a method on ApiController

You can also access the principal using the User property on ApiController.

So the following two statements are basically the same:

string id;
id = User.Identity.GetUserId();
id = RequestContext.Principal.Identity.GetUserId();