What is the difference between Invariants and Validation Rules?

Yes, I think so

In DDD, validation rules can be thought as invariants. The main responsibility of an aggregate is to enforce invariants across state changes for all the entities within that aggregate.

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Absolutely, validation is the process of approving a given object state, while invariant enforcement happens before that state has even been reached.

A corollary is that invariant enforcement is best performed by the thing that is being mutated (or created) itself, like a self-protection reflex, whereas validation is usually done by a third party.

The Always valid school of thought advocates the use of invariants over validation. I think it goes perfectly with DDD and Aggregates.