DDD Example - Version 2

Order.cs

public class Order
{
    private readonly List<OrderItem> _orderItems;

    public Order()
    {
        _orderItems = new List<OrderItem>();
    }

    public IImmutableList<OrderItem> OrderItems => _orderItems.ToImmutableList();

    public void AddOrderItem(OrderItem orderItem)
    {
        _orderItems.Add(orderItem);
    }

    public void RemoveOrderItem(OrderItem orderItem)
    {
        _orderItems.Add(orderItem);
    }
}