Entity Framework 4: AddObject vs. Attach: When to Use Each Method?

Entity Framework 4: Differences and applications of 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 handle entities that already exist in the database.

AddObject: used to create new entities

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

Attach: used to modify existing entities

In contrast, Attach is used to modify an entity that already exists in the database. EntityState is not set to Added when using Attach. It remains in the Unchanged state, indicating that no modifications have occurred since the entity was attached to the context. This allows EF to use the value of the EntityKey when calling SaveChanges to identify the 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 it from the database. For example, if you have a Person object existingPerson that already exists in the context, you can update its properties and attach it to the context:

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

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

Summary

Understanding the difference between AddObject and Attach is critical to using Entity Framework effectively. AddObject is used for newly created entities, while Attach is used for existing entities, allowing you to modify them effectively and efficiently.

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