CODEWITHSUNDEEP
c#oop
OverPalm
OverPalm

Reputation:

Overloading BindingList<T> of parent property with a subclass of T

I am having some trouble with an idea that at its simplest seems like it should work.

I am trying to overload a Property of Type BindingList<T> in a subclass with a BindingList of <subclasss T>. There are ways I can get around this but it seems the 'nicest' way would be without any direct casting. I have tried a bunch of options and have some solutions but I am not particularly happy with any of them.

Is there a best approach to this? A simple code example might be the best descriptor In this example below, I want to derive a fruitbowl to contain only apples but use the same property name to access this BindingList<> of Apples (in the case of the subclass; generic fruit in the case of the Super class).

--------Example-------

class Fruit{}
class Apple: Fruit {}

class FruitBowl
{
  protected BindingList<Fruit>  m_aFruits;

  public BindingList<Fruit> Fruits
  {
    get {return m_aFruits;}
  }
}

class AppleBowl : FruitBowl
{
  public BindingList<Apple> Fruits
  {
    get {return m_aFruits;}
  }
}

Upvotes: 2

Views: 767

Answers (2)

JaredPar
JaredPar

Reputation: 754943

What you're attempting to do is known as Co/Contra Variance. Unfortunately this is not supported on concrete types in C# (available on interfaces only for C# 4.0). Given the implementation of BindingList<T> it is not possible to do what you want and maintain only a single list.

You can attempt to fake this in several ways. One way to get around this is to only use IEnumerable<T> on the sub class. In this case a simple LINQ query will do the trick. 

class AppleBowl : FruitBowl
{
  public IEnumerableApple> TypedFruits
  {
    get {return base.Fruits.Cast<Apple>();}
  }
}

Upvotes: 2

Rex M
Rex M

Reputation: 144132

class FruitBowl<T> where T : Fruit //this requires T to inherit from Fruit
{
    protected BindingList<T> Fruits;
}

class AppleBowl : FruitBowl<Apple>
{
    //AppleBowl will have an inherited property Fruits 
    //of type BindingList<Apple>
}

Upvotes: 1

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