实体框架：Detach和AsNoTracking之间的区别

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.