public Task <IEnumerable <ValidationError> > Visit(UsageTrigger trigger) { var errors = new List <ValidationError>(); if (trigger.NumDays.HasValue && (trigger.NumDays < 1 || trigger.NumDays > 30)) { errors.Add(new ValidationError(Not.Between("Num days", 1, 30), nameof(trigger.NumDays))); } return(Task.FromResult <IEnumerable <ValidationError> >(errors)); }