In EF 4.1+, is there a difference between these 2 lines of code?
dbContext.SomeEntitySet.Add(entityInstance);
dbContext.Entry(entityInstance).State = EntityState.Added;
Or do they do the same thing? I'm wondering if one might affect child collections / navigation properties differently than the other.
When you use dbContext.SomeEntitySet.Add(entityInstance); the status for this and all its related entities/collections is set to added, while dbContext.Entry(entityInstance).State = EntityState.Added; adds also all the related entities/collections to the context but leaves them as unmodified.
So if the entity that you are trying to create has a related entity (and it's value its not null), when you use Add it will create a new object for that child entity, while with the other way it won't.
I just tested this with EF 6, with related entities/navigation properties, and in both cases the created objects were identical. (All parent and related child objects were created.) The only difference I noticed was that Add was faster by about a factor of 2. My data had 1000 parent objects, each with 5 child objects for a total of 6000 objects written to the DB.
来源：https://stackoverflow.com/questions/9082922/what-is-the-difference-between-idbset-add-and-dbentityentry-state-entitystate