When to use DbSet<T>.Add() vs DbSet<T>.Attach()

Well, when you use Attach you tell the context that the entity is already in the database, SaveChanges will have no effect over attached entities. Add, on the other hand, changes the state of the entity in the context (if it's already there) to Added, meaning it will always insert the entity in the database when you call SaveChanges.

That's the difference.

in case of ef-core

Attach is good for cases when you are adding a new entity to the database with navigational properties. Attach only marks newly created items as changed.

Let's say you are adding a new Employee to an Industry. If the industry already exists in the database it must have an ID. and the Employee you are adding is not inserted to the database yet so it does not have an ID yet (I am talking about row IDs here).

So what attach does is since the Industry already has an ID. Attach marks that as Unchanged. And your Employee who doesn't have an ID yet attach marks it as Added.

You can read more about this topic here: https://www.learnentityframeworkcore.com/dbcontext/modifying-data#attach