Dispatch.cs

public class Dispatcher
{
    public Guid Id { get; private set; }
    public string AssignedRegion { get; private set; }

    public Dispatcher(Guid id, string assignedRegion)
    {
        Id = id;
        AssignedRegion = assignedRegion;
    }
}

public class Shipment
{
    public Guid Id { get; private set; }
    public string Region { get; private set; }
    public Guid? AssignedDriverId { get; private set; }

    public Shipment(Guid id, string region)
    {
        Id = id;
        Region = region;
        AssignedDriverId = null;
    }

    public void AssignDriver(Guid driverId, Dispatcher dispatcher)
    {
        if (dispatcher.AssignedRegion != Region)
        {
            throw new UnauthorizedAccessException("Dispatcher cannot assign a load outside their assigned region.");
        }

        AssignedDriverId = driverId;
    }
}