Entity Framework: difference between Detach and AsNoTracking

Question

My goal is to copy an existing Entity, slightly modify it, and insert the modified version.

I have tried two different methods that both appear to work:

var thing = context.Things.Where(x => x.SomeID == someid).AsNoTracking().Single();
thing.AnotherID = 1234;
context.Things.AddObject(thing);
context.SaveChanges();

var thing = context.Things.Where(x => x.SomeID == someid).Single();
context.Detach(thing);
thing.AnotherID = 1234;
context.Things.AddObject(thing);
context.SaveChanges();

From what I can tell they both are accomplishing my goal. Is one of these better than the other, or are they both equally fine (or wrong!?)

Answer 1

The first version is better and I would prefer it because

• it expresses better that you don't want to track changes of the existing entity
• it doesn't attach the entity to the context in the first place while the second version attaches and then immediately detaches it (which most likely will also have slightly worse performance)
• it perserves relationships (doesn't matter in this simple example, but generally) while detaching an entity only detaches the entity itself that you pass into Detach. Related children will stay attached which comes with the price that the relationships will be cleared (a navigation collection of child entities for example would be emptied, a reference navigation property would be set to null) since EF does not allow object graphs with a mix of attached and detached entities.