Refactor my code : Avoiding casting in derived class

Question

Firstly, I feel sorry about the title, I do not know how to describe my problem exactly. I hope it will be better explained through the code.

public abstract class AB {
  public MyModel Model;
}

public class A : AB {
  public A() {
    Model = new MyModelA();
  }

  public void AMethod() {
    var model = (MyModelA) model; // I have to do this all place
  }

  public void AnotherMethod() {
    var model = (MyModelA) model; // same here
    model.NewInt = 123;
  }
}

public abstract class MyModel {

}

public class MyModelA : MyModel {
  // new properties
  public int NewInt {get;set;}
}

Take a look at the code, in order to use new properties from derived class, I have to do a cast but it is ugly when I have to use it same time all over places.

The method I think is declare another property: public MyModelA _tmp then I cast it in the constructor _tmp = (MyModelA) Model and use it instead of Model.

Are there any other appropriate ways to do this ? Thanks !

SLaks · Accepted Answer · 2011-12-25T18:47:27.120

10

You can make the base class generic:

public abstract class ServiceBase<TModel> where TModel : new() {
    protected ServiceBase() { Model = new TModel(); }
    public TModel Model { get; private set; }
}

public class AService : ServiceBase<MyModelA> {
    ...
}

edited Dec 25 '11 at 18:47

answered Dec 25 '11 at 18:41

SLaks

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Abstract class should have `protected` constructor. – bobbymcr Dec 25 '11 at 18:45
how does that help to deal with the fact that the TModel passed on to AService may have NewInt or not? – Nicolas78 Dec 25 '11 at 18:45
really nice solution, I love this :) Thanks so much ! – Dzung Nguyen Dec 25 '11 at 20:01

score 1 · Answer 2 · answered Dec 25 '11 at 18:44

1

You can maintain your Model reference in the derived class:

public abstract class AB {
  public MyModel Model;
}

public class A : AB {
      MyModel MyModel;

  public A() {
            MyModel = new MyModelA();
            Model = MyModel;
  }

  public void AMethod() {
            //just use MyModel
  }

  public void AnotherMethod() {
    MyModel.NewInt = 123;
  }
}

public abstract class MyModel {

}

public class MyModelA : MyModel {
  // new properties
  public int NewInt {get;set;}
}

answered Dec 25 '11 at 18:44

Ivo

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yea but I would assume that there's a reason he hasn't introduced a new field in the first place (ie MyModelA still has to perform a role as MaModel) – Nicolas78 Dec 25 '11 at 18:48
it looks like this is a cleaner version of my solution, without using the cast but it is still same idea. – Dzung Nguyen Dec 25 '11 at 19:59
Yes, you asked how to avoid casting :) – Ivo Dec 26 '11 at 01:20

score 0 · Answer 3 · answered Dec 25 '11 at 18:51

The solution with _tmp rids you of having to write that manual cast all the time, but the problem of a strange object design remains.

I would guess your NewInt is there to perform some sort of functionality that was also present in MyModel (otherwise you'd be better off creating a new class for that to begin with). I'm wondering if you can't encapsulate that functionality in a way that MyModelA does not have to expose anything new. This may mean changing the definition of AB in order to allow for such generalizations.

The answer, I believe, is neither syntactic nor easily found in a OOP pattern without understanding the domain. Maybe you can provide some details on that.

Ok well SLaks answer pretty nicely provides a domain-agnostic answer so my last answer may be wrong. Still, it may be worth looking into why you have to expose a new public property in MyModelA — Nicolas78, Dec 25 '11 at 18:54

Refactor my code : Avoiding casting in derived class

3 Answers3

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