public class Order
    {
        private decimal _totalAmount;
        private ISet<OrderItem> _orderItems;

        public Order()
        {
            _totalAmount = 0;
            _orderItems = new HashedSet<OrderItem>();
        }
        public virtual ISet<OrderItem> OrderItems
        {
            get { return new ImmutableSet<OrderItem>(_orderItems); }
        }

        public virtual decimal TotalAmount
        {
            get { return _totalAmount; }
        }

        public virtual void AddOrderItem(OrderItem item)
        {
            _orderItems.Add(item);
            _totalAmount = TotalAmount + item.Amount;
        }

        public virtual void RemoveOrderItem(OrderItem item)
        {
            _orderItems.Remove(item);
            _totalAmount = TotalAmount - item.Amount;
        }

    }

    public class OrderItem
    {
        public decimal Amount { get; set; }
    }

<property name="TotalAmount" access="field.camelcase-underscore"/>