Entity Framework 4: Differences and applications between AddObject and Attach methods

In Entity Framework (EF), entity management involves two methods: AddObject and Attach. AddObject is used to insert new entities into the system, while Attach is used to process entities that already exist in the database.

AddObject: Used to create a new entity

As stated in the title, AddObject is used to create a new entity. This method assigns a generated EntityKey and sets EntityState to Added. When SaveChanges is called, EF understands that this entity needs to be inserted into the database.

Attach: used to modify an existing entity

In contrast, Attach is used to modify an entity that already exists in the database. When using Attach, EntityState is not set to Added. It remains Unchanged, indicating that no modification has occurred since the entity is attached to the context. This allows EF to use the EntityKey value when calling SaveChanges to identify an entity and update or delete it as needed.

Application scenarios of the Attach method

Attach method is to update an existing entity without explicitly retrieving from the database. For example, if you have a Person object existingPerson that already exists in the context, you can update its properties and append it to the context:

var ctx = new MyEntities();
var existingPerson = ctx.Persons.SingleOrDefault(p => p.Name == "Joe Bloggs");
existingPerson.Name = "Joe Briggs";
ctx.Persons.Attach(existingPerson);
ctx.SaveChanges();

By using Attach, you can avoid executing additional queries to retrieve the current state of an entity.

Summarize

Understanding the difference between AddObject and Attach] is essential for efficient use of the Entity Framework. AddObject is used for newly created entities, while Attach is used for existing entities, allowing you to modify them efficiently and efficiently.