trx1.cs

public class ArriveHandler : IRequestHandler<Arrive>
{
    private readonly ShipmentDbContext _dbContext;

    public ArriveHandler(ShipmentDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public async Task<Unit> Handle(Arrive 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.InTransit)
        {
            throw new InvalidOperationException("Stop has already arrived.");
        }

        stop.Status = StopStatus.Arrived;
        stop.Arrived = request.Arrived;

        await _dbContext.SaveChangesAsync();

        return Unit.Value;
    }
}