Shipment.cs

public class ShipmentAggregateRoot
{
    private SortedList<int, Stop> Stops { get; } = new();

    private ShipmentAggregateRoot(IReadOnlyList<Stop> stops)
    {
        for(var x = 0; x < stops.Count; x++)
        {
            Stops.Add(x, stops[x]);
        }
    }

    public static ShipmentAggregateRoot Factory(PickupStop pickup, DeliveryStop delivery)
    {
        return new ShipmentAggregateRoot(new Stop[] { pickup, delivery });
    }

    public static ShipmentAggregateRoot Factory(Stop[] stops)
    {
        if (stops.Length < 2)
        {
            throw new InvalidOperationException("Shipment requires at least 2 stops.");
        }

        if (stops.First() is not PickupStop)
        {
            throw new InvalidOperationException("First stop must be a Pickup");
        }

        if (stops.Last() is not DeliveryStop)
        {
            throw new InvalidOperationException("Last stop must be a Delivery");
        }

        return new ShipmentAggregateRoot(stops);
    }

    public void Arrive(int stopId)
    {
        var currentStop = Stops.SingleOrDefault(x => x.Value.StopId == stopId);
        if (currentStop.Value == null)
        {
            throw new InvalidOperationException("Stop does not exist.");
        }

        var previousStopsAreNotDeparted = Stops.Any(x => x.Key < currentStop.Key && x.Value.Status != StopStatus.Departed);
        if (previousStopsAreNotDeparted)
        {
            throw new InvalidOperationException("Previous stops have not departed.");
        }

        currentStop.Value.Arrive();
    }

    public void Pickup(int stopId)
    {
        var currentStop = Stops.SingleOrDefault(x => x.Value.StopId == stopId);
        if (currentStop.Value == null)
        {
            throw new InvalidOperationException("Stop does not exist.");
        }

        if (currentStop.Value is not PickupStop)
        {
            throw new InvalidOperationException("Stop is not a pickup.");
        }

        currentStop.Value.Depart();
    }

    public void Deliver(int stopId)
    {
        var currentStop = Stops.SingleOrDefault(x => x.Value.StopId == stopId);
        if (currentStop.Value == null)
        {
            throw new InvalidOperationException("Stop does not exist.");
        }

        if (currentStop.Value is not DeliveryStop)
        {
            throw new InvalidOperationException("Stop is not a delivery.");
        }

        currentStop.Value.Depart();
    }

    public bool IsComplete()
    {
        return Stops.All(x => x.Value.Status == StopStatus.Departed);
    }
}