Shipment.cs

public sealed class Shipment
{
    public Guid Id { get; }
    public ShipmentStatus Status { get; private set; } = ShipmentStatus.Created;
    public DateTime? DispatchedAt { get; private set; }
    public DateTime? ArrivedAt { get; private set; }
    public int PalletsLoaded { get; private set; }
    public DateTime? EmptiedAt { get; private set; }

    public Shipment(Guid id) => Id = id;

    public void Dispatch(DateTime now)
    {
        if (Status != ShipmentStatus.Created) throw new InvalidOperationException("Can only dispatch from Created.");
        Status = ShipmentStatus.Dispatched;
        DispatchedAt = now;
    }

    public void Arrive(DateTime now)
    {
        if (Status != ShipmentStatus.Dispatched) throw new InvalidOperationException("Can only arrive from Dispatched.");
        Status = ShipmentStatus.Arrived;
        ArrivedAt = now;
    }

    public void Load(int pallets, DateTime now)
    {
        if (Status != ShipmentStatus.Arrived) throw new InvalidOperationException("Can only load after Arrived.");
        if (pallets <= 0) throw new InvalidOperationException("Invalid pallet count.");
        Status = ShipmentStatus.Loading;
        PalletsLoaded += pallets;
    }

    public void Empty(DateTime now)
    {
        if (Status is not (ShipmentStatus.Arrived or ShipmentStatus.Loading))
            throw new InvalidOperationException("Can only empty after Arrived/Loading.");
        Status = ShipmentStatus.Empty;
        EmptiedAt = now;
    }
}