The property 'Id' is part of the object's key information and cannot be modified

This problem happens because you are referencing the same object more than once. This is not a limitation of EF, but rather a safety feature to ensure you are not inserting the same object with two different IDs. So to achieve what you are trying to do, is simply create a new object and add the newly created object to the database.

** This issue often happens inside loops. If you are using a while or foreach loop, make sure to have the New Created Object INSIDE the loop body.

try this:

Contact contact = dbContext.Contacts.Single(c => c.contactTypeId == 1234);
contact.contactTypeId = 4;
dbContext.AddObject(contact);
dbContext.SaveChanges();

The entity that was created by the framework doesn't have a contact.ContactTypeId property. It automatically removed it and created the ContactType association inside the Contact entity.

The way to get it to work, as you suggested, is to create a ContactType object by querying the database and assigning it to contact.ContactType. For example:

Contact contact = dbContext.Contacts.Single(c => c.Id == 12345);
ContactType contactType = dbContext.ContactType.Single(c => c.Id == 3);
contact.ContactType = contactType;

Try

contact.ContactType = differentContactType;

or

contact.ContactTypeId = 3;

You are trying to set the Id of the ContactType (of the Contact) to 3.

You should add

 db.Entry(contact).State = EntityState.Detached;

After the .SaveChanges();

I had this happening when I was editing related objects from two separate contexts at the same time. Example:

DataContext ctxA = new DataContext();
DataContext ctxB = new DataContext();

Author orwell = new Author {Name = "George Orwell" };
ctxA.Add(orwell);
ctxB.Add(new Book {Name = "1984", Author = orwell});

ctxA.SaveChanges();
ctxB.SaveChanges();

My case was a little bit more convoluted (as this is obviously quite stupid) but in essence this was causing the error in my case.