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

Entity 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:

<code class="language-csharp">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();</code>

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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