fluent validation collection items not null/empty

There is a simpler approach now, using RuleForEach:

RuleForEach(model => model.TransDrops)
    .NotNull()
    .WithMessage("Please fill all items");

Make sure to use NotNull and not NotEmpty, because NotEmpty validates type's default value (in this case: int, it's 0).

You can check for more details in the documentation

First you have to use nullable integer type for collection item, otherwise empty textboxes would be bound to zero value, what makes impossible to distinguish empty textboxes and filled with zeros.

public List<int?> TransDrops { get; set; } 

Next, use predicate validator (Must rule):

RuleFor(model => model.TransDrops)
    .Must(collection => collection == null || collection.All(item => item.HasValue))
    .WithMessage("Please fill all items");

If you need to prevent empty collection being successfully validated, just add NotEmpty() rule before predicate validator: it checks that any IEnumerable not null, and have at least 1 item.