AddObject vs. Attach in Entity Framework 4: When to Use Each Method?
Jan 10, 2025 pm 01:57 PMEntity Framework 4 object management: Detailed explanation of ObjectContext.AddObject and Attach methods
Entity Framework 4 provides two methods for managing objects in a context: ObjectContext.AddObject
and ObjectSet.AddObject
for adding new objects; ObjectContext.Attach
and ObjectSet.Attach
for indicating existing objects. Although the names are similar, they play very different roles in object lifecycle management.
ObjectContext.AddObject and ObjectSet.AddObject: The birth of objects
Use ObjectContext.AddObject
and ObjectSet.AddObject
when working with new entities. These methods add objects to the context that do not yet exist in the database. The newly added entity will get a temporary EntityKey and Added EntityState. After calling SaveChanges
, the framework will recognize these objects as entities that need to be inserted into the database.
ObjectContext.Attach and ObjectSet.Attach: connection to existing objects
Unlike AddObject
, ObjectContext.Attach
and ObjectSet.Attach
are used for objects that already exist in the database. Attach
Instead of setting the EntityState to Added, it sets it to Unchanged, indicating that the object remains unchanged since being attached to the context. Entities marked with Attach
are found in the database by matching their EntityKey values, and are updated or deleted accordingly when SaveChanges
is called.
A practical application example of the Attach method
AddObject
is used to create new entities, while Attach
is useful when working with existing objects. For example, to connect an existing Person entity to an existing Address entity in the context:
var existingPerson = ctx.Persons.SingleOrDefault(p => p.Name == "Joe Bloggs"); var myAddress = ctx.Addresses.First(a => a.PersonID != existingPerson.PersonID); existingPerson.Addresses.Attach(myAddress); // 或: myAddress.PersonReference.Attach(existingPerson); ctx.SaveChanges();The
Attach
operation ensures that the Address entity is recognized as an existing entity and connected to the Person entity, allowing for correct database update or delete operations.
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