Collections, bases classes and inheritance

[Guest]

Hello,

I've got a c++ background and i'm wondering what's the best approach to this problem.

I've got a base class Animal with a set of minimal properties for an animal. From animal i've
derived a Dog and a Cat class which are extended with specific properties of it's species. Now i want
a collection to store these animals in. I've created an Collection derived from CollectionBase (see code).

But when i add a Dog or a Cat to this collection i get an exception even when i cast the cat or dog back
to it's base class.

Is there a collection to solve this problem. Which can take the base class

using System;
using System.Collections;

namespace Petshop
{
public class AnimalCollection : CollectionBase
{
public Animal this[ int index ]
{
get { return( (Animal) List[index] );}
set { List[index] = value;}
}

public int Add( Animal value )
{
return( List.Add( value ) );
}

public int IndexOf( Animal value )
{
return( List.IndexOf( value ) );
}

public void Insert( int index, Animal value )
{
List.Insert( index, value );
}

public void Remove( Animal value )
{
List.Remove( value );
}

public bool Contains( Animal value )
{
// If value is not of type Animal, this will return false.
return( List.Contains( value ) );
}

protected override void OnInsert( int index, Object value )
{
if (value.GetType() != Type.GetType("Petshop.Animal"))
{
throw new ArgumentException( "value must be of type Animal.", "value" );
}
}

protected override void OnRemove( int index, Object value )
{
if (value.GetType() != Type.GetType("Petshop.Animal"))
{
throw new ArgumentException( "value must be of type Animal.", "value" );
}
}

protected override void OnSet( int index, Object oldValue, Object newValue )
{
if (newValue.GetType() != Type.GetType("Petshop.Animal"))
{
throw new ArgumentException( "newValue must be of type Animal.", "newValue" );
}
}

protected override void OnValidate( Object value )
{
if (value.GetType() != Type.GetType("Petshop.Animal"))
{
throw new ArgumentException( "value must be of type Animal." );
}
}
}
}

Cheers

Erik

ps. No cats and dogs were harmed during testing of this code.

 

[Mattias Sjögren]

Erik,

if (value.GetType() != Type.GetType("Petshop.Animal"))
{
throw new ArgumentException( "value must be of type Animal.", "value" );
}

Replaceing that with

if ( !(value is Petshop.Animal) ) ...

should get rid of the exceptions.



Mattias

 

[Mattias Sjögren]

No that won't work.

Did you try it?

The passed value is still a cat or a dog even after i cast the object to the base class in the Add methode.

Of course, casting from one reference type to another doesn't change
the actual object's type, only the type of reference you have to it
and which part of its interface you see.

so i still get the exception.

Where exactly (on which line) do you get the exception?



Mattias