Question

I need to switch data context for some records. So basically I have db context A and B, I fetch records using A, then I switch to B, alter records, and save them.

When I call Attach for B, I get exception that records are using by multiple data context, when I add Detach for A, I get exception, that records are not attached to A.

So how can I switch the data context?

Example

db_creator is creator of db context. Here I fetch the data (corrected version):

using (var db = db_creator.Create())
{
  var coll = db.Mailing.Where(it => !it.mail_IsSent).ToList(); // (*)
  coll.ForEach(it => db.Detach(it));
  return coll;
}

(*) the mistake was caused by refactoring this piece, I created extra data context, and then later I tried to detach records from another one.

Now I would like to switch data context to new one, do some computation and modifications and save the records. coll is List of the records:

using (var db = db_creator.Create())
{
  coll.ForEach(it => db.Mailing.Attach(it));
  ...
  db.SaveChanges();
}

Answer 1

I recommend change your design and have ONE context at a time. (Based on your project type this could vary. Usually in web apps it’s one context per http request.)

For example in a web application, you can do this like below:

protected MyContext Context
{
    get
    {
        var context = HttpContext.Current.Items["MyContext"];
        if (context == null)
        {
            context = new MyContext();
            HttpContext.Current.Items.Add("MyContext", context);
        }
        return context as MyContext;
    }
}  

And dispose it in your Application_EndRequest:

app.EndRequest += (sender, args) =>
{
    HttpContext.Current.Items.Remove("MyContext");
}

If you have multiple project types, then consider using an Ioc.
But if you still want to use two contexts, you can do as below(myEntity is your object you want to detach/attach):

if (context1.Entry(myEntity).State != EntityState.Detached);
{
    ((IObjectContextAdapter)context1).ObjectContext.Detach(myEntity);
}
context2.MyEntities.Attach(myEntity);