Problem: I am trying to update a List. If a certain item's ID already exists in the List, I want to add onto that item's quantity. If not, then I want to add another item to the list.

            cart = (List<OrderItem>)Session["cart"];

            for(int counter = cart.Count-1; counter >= 0; counter--)
            {
                if (cart[counter].productId == item.productId)
                {
                    cart[counter].productQuantity += item.productQuantity;
                }
                else if (counter == 0)
                {
                    cart.Add(item);
                }
            }

"cart[counter]" and "item" represent an instance(s) of a custom object of mine. Currently when I finally find a matching ID, everything APPEARS as though it should work, but I get a StackOverflowException thrown in my custom object class.

    public int productQuantity
    {
        get
        {
            return _productQuantity;
        }
        set
        {
            productQuantity = value;
        }
    }

It gets thrown right at the open-bracket of the "set". Could somebody please tell me what the heck is wrong because I've been going at this for the past 2+ hours to no avail. Thank you in advance.