How to read the Custom Fields of a User, not assigned as a resource with CSOM?

I don't know how to read the custom fields of a User (not assigned as a resource).

If a user is a resorce, I can access to the custom fields, with the following code (just an example):

 var cfList = Mycontext.EnterpriseResources.FirstOrDefault().CustomFields

but whether a resource is just a user, how can I access to the custom fields informations?

Thank you