Inheriting a base class - Invalid Cast Exception

[Chet Cromer]

I am creating a set of base classes and sub classes to use throughout a
program I'm developing. The base class represents a generic "lookup
table" from my database that contains lists of things like
manufacturers, makes, modes, etc. of cars. I have created a generic
"datacollection " class and a generic "dataobject " class to represent the
table and the rows within that table as a collection of objects with
generic properties for manipulating fields, adding rows, etc.

The problem I'm having is that I have a generic "Item" method in the
DataCollection class that takes a key parameter and then looks it up in
the data table and returns the information as a "DataObject " class. I
have then written subclasses to more specifically use the base class for
a particular type of data (in the example I'm providing, a vehicle
manufacturer). So I now have a "Manufactur ers" object that inherits from
DataCollection and a "Manufactur er" object that inherits from
DataObject. So, back to my problem. In the item method, the generic
DataCollection class requires a "Key" that will be used for the lookup.
It then searches its internal data table and returns the appropriate
DataObject object. However, what I WANT, is to create an overriding Item
object in my Manufacturers object that returns a MANUFACTURER instead of
a DATAOBJECT. So I created a Public Shadows property that returns a
Manufacturer object but callse the MyBase.Item function to get it. But
MyBase.Item returns a DataObject, not a objManufactuer, and then I get
thrown my Invalid Cast Exception.

I hope that explains my predicament. Can anyone help? Here is the basics
of my code:

Public Class objDataCollecti on

Public Readonly Property Item (byval Key as Object) as
objDataObject

Get

...

Return new objDataObject(. ..)

End Get

End Property

End Class

Public Class objDataObject

..

End Class

Public Class objManufacturer s

Inherits objDataCollecti on

Public Shadows Readonly Property Item(byval Code as String)
as objManufacturer

Get

return MyBase.Item(cod e)

End Get

End Property

End Class

Public Class objManufacturer

Inherits objDataObject

..

End Class

[Chet Cromer]

I'm back to my inheritance problem again, with a new twist.

My DataCollection object exposes a readonly "collection " property that
returns a collection of DataObject objects.

I want my Manufacturers object (a subclass of my DataCollection object)
to return a collection of Manufacturer object, but since the code for
creating the collection is in my base class, it doesn't know what type
of objects the subclass will be wanting to the code creates a collection
of DataObject objects... and then when I try to loop through them as
Manufacturers to get the extra functionality of the Manufacturer object,
I cannot.

Here is my current code to create the collection. This is part of the
base class DataCollection:

Public Class DataCollection
Public Readonly Property Collection() as ICollection
Get
'Gets an array of rows to build the collection from
Dim Rows() as DataRow = mDataTable.Sele ct()
Dim Row as DataRow
Dim Col as New Collection
For Each Row in Rows
'Create an object, the constructor utilizes the current row
Dim obj as New DataObject(row)
Col.Add(obj,obj .Key)
Next Row
Return Col
End Get
End Property
End Class

Here then is the code I want to use to loop through each manufacturer

Dim Mans as new Manufacturers
Dim Man as Manufacturer
For each Man in Mans.Collection
Msgbox Man.Manufacture rName
Next Man

The problem is that Mans.Collection returns a collection of DataObject
objects, not Manufacturer objects. I cannot figure out how to tell the
Collection property of the DataCollection object to create a collection
of differently types objects depending on what is inheriting it, because
of course it doesn't know. The only thing I can think of doing is
creating an Overriding property in the Manufacturers object like this:

Public Overrides Readonly Property Collection() as ICollection
Get
Dim ColNew as New Collection
Dim DO as DataObject
For Each DO in MyBase.Collecti on
ColNew.Add DO.Key,New Manufacturer(do )
Next DO
Return ColNew
End Get
End Property

This code loops through all the DataObjects in the collection and
transforms them into Manufacturer objects using the code we talked about
earlier where I pass the DataObject to the constructor of my
Manufacturer object.

Are there any better ways around this? One that WON'T have me writing a
block of code every time I need to create a collection property, which
is part of the reason I'm trying to create a base class for.