DDD - Example - Version 1

Order.cs

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

    public IReadOnlyCollection<OrderItem> OrderItems
    {
        get => (IReadOnlyCollection<OrderItem>)_orderItems;
        private set => _orderItems = (ICollection<OrderItem>)value;
    }

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

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