How to attach an EF Object to a new Context in c#

Question

I am experiencing the pain in the A** of EF and according too google, I am not the only one with this issue. I Have a list of object that I got from EF and then I set this list as the DataContext (WPF) of an itemscontrol. Since, after the "using" block the context is destroyed, I would have to make a new query and object with the same ID When wishing to save the changes. I try to attach it to the Context, I try saving the changes, but they are not stored.

Some people have tried several things, but does maybe someone know of a handy and smart solution for this problem?

I tried Attach(), AttachTo(), nothing worked

P.S. It's EF 4.0

EDITED - SOLVED

Adding the following line of code between attaching and saving solved everything in EF 4.0

_context.ObjectStateManager.ChangeObjectState(modifiedEntity, System.Data.EntityState.Modified);

in EF 4.1

_context.Entry(modifiedEntity).State = System.Data.EntityState.Modified;

Answer 1

When you do an Attach() of an Entity Framework object to a new DbContext, it is entered with State EntityState.Unchanged. This happens regardless of whether or not the properties of the object have actually been changed since it was originally fetched from the DB (in a different context).

To tell EF about changes that happened outside of the current DbContext, you simply need to set the State of the object to EntityState.Modified before calling DbContext.SaveChanges().

        Foo foo=null;
        using (var db = new MyContext())
        {
            db.Foos.Add( new Foo { MyValue = OLD_VALUE } );
            db.SaveChanges();   // foo written to DB with MyValue = OLD_VALUE

            foo = db.Foos.FirstOrDefault(); // grab foo
        }

        // leave context and update foo...
        foo.MyValue = NEW_VALUE;

        using (var db = new MyContext())
        {
            db.Foos.Attach(foo);    // foo is attached in the 'unchanged' state...
            db.SaveChanges();       // ...so this statement has no effect

            // At this point, db.Foos.FirstOrDefault().MyValue will be NEW_VALUE, yet
            // the "real" value of the object in the DB is OLD_VALUE.

            db.Entry(foo).State = EntityState.Modified; // setting foo to "Modified" will cause...
            db.SaveChanges();                           // ...foo to be written out with NEW_VALUE
        }

Answer 2

Honestly, this is a bit of a pain. Here is a post I wrote about attaching a modified object when I had a similar problem, there are others, perhaps it helps...

Entity Framework Modify Detached Object

So, to avoid the irritation, this is what I normally do:

public ActionResult Edit(Category cat)
{
  //cat is the modified object
  using(MyEFEntities db = new MyEFEntities())
  {
    var dbCategory = db.Categories.FirstOrDefault(o => o.ID == cat.ID);
    dbCategory.Description = cat.Description.
    dbCategory.Value = cat.Value;
    db.SaveChanges();
  }
}

So, as you can see, rather than messing with attaching and such, I just get the object in the current context, modify it, then save. It truly is probably better to just attach your modified object to the context correctly and then save, but I get annoyed with how unobvious (to me) it is to do.

Maybe someone else will provide a more correct solution for you.

Good Luck.