C# Pattern for abstract class specific code

Question

I have an abstract "Action" class, which has derived types of ActionAppointment, ActionCall, ActionEmail, and ActionLetter. I'm trying to write a function that will DRY up our service layer, so we're not writing CRUD calls 5 times anymore.

I have in our service layer some update logic (lots of other code removed for brevity):

private IServiceResponse UpdateAction<T>(T action, string originalActionStatus) where T : Action
{
        if (action.GetType() == typeof(Action))
        {
            _actionRepository.Update(action);
        }
        else if (action.GetType() == typeof(ActionAppointment))
        {
            _actionAppointmentRepository.Update(action as ActionAppointment);
        }
        else if (action.GetType() == typeof(ActionCall))
        {
            _actionCallRepository.Update(action as ActionCall);
        }
        else if (action.GetType() == typeof(ActionEmail))
        {
            _actionEmailRepository.Update(action as ActionEmail);
        }
        else if (action.GetType() == typeof(ActionLetter))
        {
            _actionLetterRepository.Update(action as ActionLetter);
        }
}

Unfortunately, how our repositories are setup, I have to use the specifically named repositories (ie. I can not update an ActionLetter through the _actionRepository even though it derives from Action)

I have been reading on different patterns, and it sounds like something similar to a Factory Pattern, but I can't see how to make it work.

Am I missing something stupid?

You might consider the *Visitor* pattern (http://en.wikipedia.org/wiki/Visitor_pattern). — David R Tribble, Jun 23 '11 at 20:14
Yes a factory pattern is a solution. Your Repositories must either have a common base class or implement an interface. They must all have an Update method that accepts the object typed as Action. The factory accepts the type as a parameter (in whatever form, name, type, enum if one is available) and returns the correct repository. That is pretty much it. Is there a specific point of confusion on how to implement the Factory? — Sisyphus, Jun 24 '11 at 05:02
I'm unsure how to implement a factory pattern for this one, since IRepository is generic based on each domain model object. — mandreko, Jun 25 '11 at 02:45
Sorry for slow response. A factory will not work with generics they are not type polymorphic. Generics are based on parametric polymorphism. It is a functional scope polymorphism. Factories require class scope polymorphism. Not sure I understand what you get out of this design beyond more type safety for basic updates etc? I use nHibernate it has an untyped Update anyway. I opt for a base Repository class with a common Update, and add some minimal typing - all my persisted objects have a single common base class, so i can't pass e.g net type as object that is not valid for persisting. — Sisyphus, Jun 28 '11 at 08:42

Matti Virkkunen · Accepted Answer · 2011-06-23T20:15:00.277

11

Can't you just write an overload of that method for each action type? Forget the <T> and typeof stuff - what you're doing is implementing a built-in language feature (method overloading) by hand, and in a fragile way too.

edited Jun 23 '11 at 20:15

answered Jun 23 '11 at 20:07

Matti Virkkunen

63,558
9
127
159

3

Yes, that is exactly what polymorphism and inheritance is all about. – David R Tribble Jun 23 '11 at 20:12
The code was previously written as a method for each Action type. I am just trying to DRY it up some, but I guess maybe I can DRY up some of the shared components, but have the specific repository code in each one. – mandreko Jun 25 '11 at 02:38
@Matt: Your if-else contraption isn't making the code any DRYer, that's for sure. – Matti Virkkunen Jun 25 '11 at 13:33

score -2 · Answer 2 · answered Jun 23 '11 at 20:15

-2

Let's inverse the logic here:

abstract class Action {
    protected abstract Repository GetRepository();
    protected void Update(){
       this.GetRepository().Update(this);
    }
}

All you have to do is override the GetRepository in each of the deriving classes. For example:

class ActionAppointment : Action {
    protected override Repository GetRepository() {
        return _actionAppointmentRepository;
    }
}

answered Jun 23 '11 at 20:15

linepogl

9,147
4
34
45

1

not a very good idea for a model class to know about how its stored. – Daniel A. White Jun 23 '11 at 20:17
2

-1, Injecting services (even repositories) into domain objects is a bad practice. – smartcaveman Jun 23 '11 at 20:17
Agreed with smartcaveman. It violates our rules to have that type of dependency. – mandreko Jun 25 '11 at 02:35

C# Pattern for abstract class specific code

2 Answers2