An entity object cannot be referenced by multiple instances of IEntityChangeTracker. with EF6

Question

These are my classes

public class Bill
    {
        [Key]
        public int BillID { get; set; }

        [Required]
        public int BillNumber { get; set; }

        [Required]
        public DateTime Date { get; set; }

        [Required]
        public string UserName { get; set; }

        [Required]
        public string Name { get; set; }

        [Required]
        public string Adress { get; set; }

        [Required]
        public string PostalCode { get; set; }

        [Required]
        public string City { get; set; }

        [Required]
        public string Country { get; set; }

        public Bill()
        {
            Date = new DateTime();
            Date = DateTime.Now;
        }



public class Cart
    {
        [Key]
        public int CartID { get; set; }

        [Required]
        public int BillID { get; set; }

        [Required]
        [ForeignKey("BillID")]
        public virtual Bill Bill { get; set; }

        public virtual ICollection<CartItems> Products {get; set;}
    }

public class CartItems
    {
        [Key]
        public int CartItemID { get; set; }

        [Required]
        public Product Product { get; set; }

        [Required]
        public int Qunatity { get; set; }
    }
}

I am buidlding an ASP.NET 4 web application. I have a problem when I want to add products to the cart, it throws an error

An entity object cannot be referenced by multiple instances of IEntityChangeTracker.

when I want to add a cart obejct to the Carts in context.

Bill bill = new Bill();
            bill.BillNumber = Bill.GetLastBillNumber(context) + 1;
            bill.Adress = txtAdress.Text;
            bill.City = txtCity.Text;
            bill.Country = ddlCountry.SelectedItem.Value.ToString();
            bill.Name = txtName.Text;
            bill.PostalCode = txtPostalCode.Text;
            bill.UserName = lblUserName.Text;

            context.Bills.Add(bill);
            context.SaveChanges();

            Cart cart = new Cart();
            cart.Bill = bill;
            cart.BillID = bill.BillID;
            cart.Products = Application["CartProducts"] as List<CartItems>;

            context.Carts.Add(cart);
            context.SaveChanges();

I read about the problem, but I cant do the Detach its not working for me;

the DataContext is definied as privet in the class

private DataContext context = new DataContext();

If anyone can help me please.

Thx.

Answer 1

Did you load the Products in Application[CartProducts] somewhere else, with another context? In that case that's the problem.

A DataContext is a Unit of Work implementation, which means that all the work you do in a single operation has to be done on the same context.

While it is generally a good idea to cache the products, it won't work. You will have to create a copy of the objects in the list instead.

cart.Products = (Application["CartProducts"] as List<CartItems>).Select(ci =>
                 CartItem { ProductId = ci.ProductId, Quantity = ci.Quantity }).ToList();

Note that I have replaced Product with ProductId to avoid having to step one more level into the object tree. In the database, the reference between the objects will be implemented as a foreignkey and by mapping that to ProductId you will be able to access it and use it directly. See this blogpost of mine for details about mapping the foreign key.