how should i add an object into a collection maintained by aggregate root

Question

lets say i have a BlogPost aggregate root. it holds a List<Comment>.
how should the BlogPost AddComment signature look? is it OK to use:

public void AddComment(Comment comment)
{
    Comments.Add(comment);
}

or should i avoid creating references to root's children outside of it, and do something like this:

public void AddComment(string text, string email)
{
    Comment comment = new Comment(text, email);
    Comments.Add(comment);
}

Answer 1

If you believe in DDD, it's perfectly fine to know about some entity beneath the aggregate root, as long as you do not store an ID or a reference to it somewhere outside of the aggregate.

I would go for the blogPost.AddComment(new Comment(...))-version.

Answer 2

If you consider Comment to be an aggregate of BlogPost and to not make sense out of that scope then you should be using the second example.

The aggregate root should control how the aggregates are instantiated so their constructors should not be visible outside of the aggregate root.

Plus, Comment should be a child class of BlogPost if you want a true relation of AggregateRoot-Aggregate.