Question

I have a Base Class, called primitive Graphics. derived from this class are several different types of graphics, squares, rectangles, lines, etc.

I am storing those graphics in an object that inherits collectionbase. This causes a problem because I cannot access any of the members in the derived class when they are added to the collection. Here is the default property for my primitivecollection class

Public Class PrimitiveCollection     Inherits CollectionBase     ''' <summary>      ''' Get or set a primitive object by index      ''' </summary>      Default Public Property Item(ByVal index As Integer) As Primitive         Get             Return DirectCast(List(index), Primitive)         End Get         Set(ByVal value As Primitive)             List(index) = value         End Set     End Property 

My current workaround is to just put all of the public members in the base class, however this is starting to look ugly as I add more derived classes that need members available to the derived class only

Answer 1

Your question is about inheritance and interfaces. The following is my opinion

Each Primitive should implement an interface, IPrimitive. Every Primitive in the Collection implements IPrimitive and the only things in IPrimitive are the things that apply to all types of IPrimitives. For example: Draw()

So when working with your collection, you have a collection of drawable objects. If you intend to work with a collection as a collection of just Rectangles, you should use a List<Rectangle> rather than a collection of Primitives.

Putting more properties onto the base class (or IPrimitive) is a bad idea, because they won’t make sense for all objects. Width could work for a Rectangle, but gets shady when you’re working with a Trapezoid or Triangle. Instead the Commonality should be as little as is necessary to work with the objects as a collection. And when you need to work with the objects as their derived classes, you should be referencing them without using the collection.