如何将列表重新附加到DbSet？

Question

When dealing with Code First (EF 4.3), is there a way to work with a List<T> from a DbSet<T> , and then persist the changes to the list?

For example...

class Program {
    static void Main(string[] args) {
        Database.SetInitializer(new DropCreateDatabaseAlways<Context>());
        Database.DefaultConnectionFactory = new SqlCeConnectionFactory("System.Data.SqlServerCe.4.0");

        using (Context c = new Context()) {

            // Works
            c.Entities.Add(new Entity());

            // Doesn't work
            List<Entity> entities = c.Entities.ToList();
            entities.Add(new Entity());

            // Somehow attach the new unattached elements?

            c.SaveChanges();

            Console.WriteLine(c.Entities.Count()); // Prints 1
            Console.ReadLine();
        }
    }
}

class Context : DbContext {
    public DbSet<Entity> Entities { get; set; }
}

class Entity {
    public int Id { get; set; }
    public int Foo { get; set; }
}

Is there a way that I can do this?

Answer 1

In this trivial example, you could do something like this.

foreach(Entity entity in entities)
{
     var entry = c.Entry(entity);

     if (entry.State == EntityState.Detached)
     {
         c.Entities.Add(entry);
     }
}

However, this probably won't work in a more complex scenario. There are a couple of different ways to solve this.

1. If the Id is auto assigned from the database, you could check if Entity.Id == 0 , assuming that Id starts at 1.
2. If you are assigning the Id manually, you could query the table to see if the Id does not exist.
3. Keep track, somehow, you decide, of which records were added after the fact. This could be through a second list. You could have your own State property which does not map to the database. This is easier if you project your entities on to application specific models.
4. Or you could just add it to your list and the context at the same time.