Pickup.cs

public class PickupHandler : IRequestHandler<Pickup>
{
    private readonly ShipmentDbContext _dbContext;
    private readonly IBus _bus;

    public PickupHandler(ShipmentDbContext dbContext, IBus bus)
    {
        _dbContext = dbContext;
        _bus = bus;
    }

    public async Task<Unit> Handle(Pickup request, CancellationToken cancellationToken)
    {
        var stop = await _dbContext.Stops.SingleOrDefaultAsync(x => x.StopId == request.StopId);

        if (stop == null)
        {
            throw new InvalidOperationException("Stop does not exist.");
        }

        if (stop.Status == StopStatus.Departed)
        {
            throw new InvalidOperationException("Stop has already departed.");
        }

        // If they haven't arrived yet, let's allow them to perform this action, and we'll just set the
        // Arrived datetime to our departed DateTime
        if (stop.Status == StopStatus.InTransit)
        {
            // Gotcha! Now we've made a state change without publishing an Arrived event.
            stop.Arrived = request.Departed;
        }

        if (request.Departed < stop.Arrived)
        {
            throw new InvalidOperationException("Departed Date/Time cannot be before Arrived Date/Time.");
        }

        stop.Status = StopStatus.Departed;
        stop.Departed = request.Departed;

        await _dbContext.SaveChangesAsync();

        await _bus.Publish(new PickedUp(request.StopId, request.Departed));

        return Unit.Value;
    }
}