public class DefaultController : Controller
{
    protected readonly DatabaseContext _context;
    protected readonly User _user;

    public DefaultController(DatabaseContext context)
    {
        _context = context;

        if (HttpContext.Request.RequiresAuthorization) // pseudo code
        {
            var id = User.FindFirst(ClaimTypes.NameIdentifier).Value;
            _user = _context.Users.Find(id);
        }
    }
}