Question

I have an entity structure as follows:

IManager: IDeletable
{
IEnumerable<IFund> Funds {get;}
IFailureNotification Delete();
}

IFund : IDeletable
{
IEnumerable<IFundClass> FundClasses
IFailureNotification Delete();
}

IFundClass: IDeletable, IInvestable
{
IFailureNotification Delete();
}

And I have a service which takes an IDeletable and calls Delete on it. Depending on the return value it then either commits the transaction or rolls it back. I’m using NHibernate to persist the classes so can’t put RI in the DB and catch the exception (which I wouldn’t like anyway).

This is a classic case for polymorphism and the Manager loops through its Funds and deletes them before deleting itself, the Fund in turn delete the FundClasses before deleting themselves, so the service can just take any entity implementing IDeletable and know that the deletion will perform the appropriate actions at all levels.

Here’s the problem: The fund classes need to find if they’re being used in a completely separate context using the IInvestable interface which they don’t know anything about. This requires a service – the IInvestmentCalculationService.

Obviously I don’t want to inject the InvestmentCalculationService into the fund class entity constructor and I don’t want to inject it into the delete method as this is on Funds and Managers as well as many other classes so doesn’t make any sense – also means that as soon as we have more requirements we have to change the delete methods on everything.

I’m a bit tempted by the Domain Events model here: http://www.udidahan.com/2009/06/14/domain-events-salvation/ but I’m not confident that it’s right as I’m trying to get data back from the triggered event handler – which would work but smells a little bit wrong, and all the examples only show fire and forget situations.

Does anyone have any suggestions?

Answer 1

Thanks to all for their help, I was really impressed by the audience I attracted! I especially liked mcintyre’s philosophy and it’s genuinely affected my thinking since. In this case though we went for double dispatch. Feels a little more stable.

Cheers