Entity Framework 4 – AddObject vs Attach

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ObjectContext.AddObject and ObjectSet.AddObject:

The AddObject method is for adding newly created objects that do not exist in the database. The entity will get an automatically generated temporary EntityKey and its
EntityState will be set to Added. When SaveChanges is called, it will be clear to the EF that this entity needs to be inserted into the database.

ObjectContext.Attach and ObjectSet.Attach:

On the other hand, Attach is used for entities that already exist in the database. Rather than setting the
EntityState to Added, Attach results in an Unchanged EntityState, which means it has not changed since it was attached to the context. Objects that you are attaching are assumed to exist in the database. If you modify the objects after they’ve been attached, when you call SaveChanges the value of the EntityKey is used to update (or delete) the appropriate row by finding its matching ID in the db table.

Furthermore, using the Attach method, you can define relationships between entities that already exist in the ObjectContext but that have not been connected automatically. Basically the main purpose of Attach, is to connect entities that are already attached to the ObjectContext and are not new so you cannot use Attach to attach entities whose EntityState is Added. You have to use Add() in this case.

For example, let’s assume your Person entity has a navigation property named Addresses which is a collection of Address entity. Let’s say you have read both Objects from context, but they are not related to each other and you want to make it so:

var existingPerson = ctx.Persons.SingleOrDefault(p => p.Name = "Joe Bloggs" };
var myAddress = ctx.Addresses.First(a => a.PersonID != existingPerson.PersonID);
existingPerson.Addresses.Attach(myAddress);
// OR:
myAddress.PersonReference.Attach(existingPerson)
ctx.SaveChanges();

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