New Post: Validating Collection of Primitive Types

I think rhe reason you are getting error there is that domains.Any() does not have any lambda in it like this: domains.Any(x => x.whatever == "whatever")

I am not sure where you were going with that Any...

You could do something like this:

RuleFor(x => x.Domains)
    .Cascade(CascadeMode.StopOnFirstFailure)
    .NotNull()
    .WithMessage("Domains cannot be null")
    .SetCollectionValidator(new DomainNameCollectionValidator());