Dispose DbContext not dispose the ObjectContext entities - asp.net

Question

I'm using EF 5.0 to create a web and I have some issues disposing my context. All the times that I use a context is inside a using sentence, so the context should be disposed automatically but in a specific moment I get the next error when I try to attach an entity to a context:
An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.
It semms that the entity is not disposed. How is the way to manage this situation? Do I have to dispose the ObjectContext to dispose the entities or is there any way to check if the Entity is attached?
Regards.

Answer 1

One way to do it is to detach the existing object before attaching. I don't have VS in front of me so I apologize if the code isn't exactly correct.
var existingObject = dbContext.Users.Local
.FirstOrDefault(x => x.id = newObject.id);
if (existingObject != null)
{
// remove object from local cache
dbContext.Entry(existingObject).State = EntityState.Detached;
}
dbContext.Users.Attach(newObject);
In case this doesn't fix the problem, you'll have to go to the old way of detaching objects.
// remove object from local cache
ObjectContext objectContext = ((IObjectContextAdapter)dbContext).ObjectContext;
objectContext.Detach(existingObject);

Answer 2

If you do something like that:
User u;
using (Entities ent = new Entities())
{
u = ent.Users.Single(a => a.ID == 123);
}
using (Entities ent2 = new Entities())
{
//loading the same user
User user2 = ent2.Users.Single(a => a.ID == 123);
//trying to attach the same object with the same key
ent2.Attach(u);
}
then you will get this error (I haven't tested this code).
EDIT: one of the solutions is to change the object's state:
ent2.Attach(u);
ent2.ObjectStateManager.ChangeObjectState(u, EntityState.Modified);
another solution is to check if the entity is already attached:
ObjectStateEntry state = null;
if(!ent2.ObjectStateManager.TryGetObjectStateEntry(((IEntityWithKey)u).EntityKey, out state))
{
ent2.Attach(u);
}

Answer 3

Dispose doesn't mean "reset to factory settings". It is a way to clean up unmanaged resources like database connections and such.

Answer 4

The problem has nothing to do with disposing a context or not. It even has nothing to do with having multiple contexts somewhere in place. If this would be the problem you would get the "An entity object cannot be referenced by multiple instances of IEntityChangeTracker" exception which is totally different to your exception.
You can simulate your exception quite easily with only a single context:
using (var ctx = new MyContext())
{
var customer1 = new Customer { Id = 1 };
var customer2 = new Customer { Id = 1 }; // a second object with the same key
ctx.Customer.Attach(customer1);
ctx.Customer.Attach(customer2); // your exception will occur here
}
The problem causing this exception is normally more hidden, expecially if you keep in mind that attaching or setting a state (for example to Modified) will also attach all related entities in the object graph of the entity you are attaching. If in this graph are two objects with the same key you'll get the exception as well, although you didn't attach those related entities explicitly.
But it's impossible to find the exact reason without more details about your code.