C# 4 and .NET 4

882 ❘ ChaPTer 31 AdO.net entity frAmewOrk
}
}
var r = entry.Entity as Racer;
if (r != null)
{
Console.WriteLine("{0}: {1}", state, r.Lastname);
}
code snippet Formula1Demo/Program.cs
When you run the application, the added and modified racers are displayed, and the properties changed
with their original and current values are shown:
Added: Alguersuari
Modified: Alonso
state of Fernando: Modified
modified: Starts
original: 138
current: 139
attaching and detaching entities
When returning entity data to the caller, it might be important to detach the objects from the object
context. This is necessary, for example, if an entity object is returned from a Web service. Here, if the entity
object is changed on the client, the object context is not aware of the change.
With the sample code, the Detach() method of the ObjectContext detaches the entity named fernando
and thus the object context is not aware of any change done on this entity. If a changed entity object is
passed from the client application to the service, it can be attached again. Just attaching it to the object
context might not be enough because it doesn’t give the information that the object was modified. Instead,
the original object must be available inside the object context. The original object can be accessed from the
store by using the key with the method GetObjectByKey() or TryGetObjectByKey(). If the entity object
is already inside the object context, the existing one is used; otherwise it is fetched newly from the database.
Invoking the method ApplyCurrentValues() passes the modified entity object to the object context,
and if there are changes, then the changes are done within the existing entity with the same key inside
the object context, and the EntityState is set to EntityState.Modified. Remember that the method
ApplyCurrentValues() requires the object to exist within the object context; otherwise the new entity
object is added with EntityState.Added:
using (var data = new Formula1Entities())
{
data.ObjectStateManager.ObjectStateManagerChanged +=
ObjectStateManager_ObjectStateManagerChanged;
ObjectQuery racers = data.Racers.Where("it.Lastname='Alonso'");
Racer fernando = racers.First();
EntityKey key = fernando.EntityKey;
data.Racers.Detach(fernando);
}
// Racer is now detached and can be changed independent of the object context
fernando.Starts++;
Racer originalObject = data.GetObjectByKey(key) as Racer;
data.Racers.ApplyCurrentValues(fernando);
code snippet Formula1Demo/Program.cs
storing entity Changes
Based on all the change information with the help of the ObjectStateManager, the added, deleted, and
modified entity objects can be written to the store with the SaveChanges() method of the ObjectContext
class. To verify changes within the object context, you can assign a handler method to the SavingChanges
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