DDD Example - Version 1

OrderTests.cs

public class OrderTests
{
    [Fact]
    public void OrderItemsShouldNotBeManipulatedOutsideOrder()
    {
        var order = new Order();
        order.AddOrderItem(new OrderItem(1, "Laptop"));
        order.AddOrderItem(new OrderItem(2, "Keyboard"));

        Assert.True(order.OrderItems.Count == 2);

        var orderItem = new OrderItem(3, "Mouse");

        var orderItems = (ICollection<OrderItem>)order.OrderItems;
        orderItems.Add(orderItem);

        Assert.True(order.OrderItems.Count == 2);
    }
}