Inherit a class to remove a property and change method logic - asp.net

Question

The following class is used in an ASP.NET application to read currencies from a database result set and add them up (e.g. show totals in US Dollars plus show totals in GB Pounds). It works in the following manner:
Read currency ID value
If currency ID exists already, increase the total for that currency
If currency ID does not exist, add it to the list with its value
Next
It works well using the CurrencyID property as the differentiator between each unique currency. However, it has now become apparent that IsoCurrencySymbol is also unique for each currency by default, and so CurrencyID is not actually needed.
So... I was wondering if it would be possible to inherit from this class and remove any reference to CurrencyID, therefore making the CompareTo method use IsoCurrencySymbol instead.
The trick is to leave the existing class as it is used extensively, but introduce a modified version without CurrencyID being needed. Is this possible to do please?
<Serializable()> _
Public Class CurrencyCounter
<Serializable()> _
Private Class CurrencyType
Implements IComparable
Public IsoCurrencySymbol As String
Public CurrencySymbol As String
Public CurrencyID As Int16
Public Amount As Decimal
Public Function CompareTo(obj As Object) As Integer Implements System.IComparable.CompareTo
If Not TypeOf (obj) Is CurrencyType Then
Throw New ArgumentException("Object is not a currency type")
Else
Dim c2 As CurrencyType = CType(obj, CurrencyType)
Return Me.CurrencyID.CompareTo(c2.CurrencyID)
End If
End Function
End Class
Private _Currencies As List(Of CurrencyType)
Public Sub New()
_Currencies = New List(Of CurrencyType)
End Sub
Private Sub AddStructToList(CurrencyID As Integer, IsoCurrencySymbol As String, CurrencySymbol As String, Amount As Decimal)
If IsoCurrencySymbol <> String.Empty AndAlso Amount > 0 Then
Dim s As New CurrencyType
s.CurrencyID = CurrencyID
s.IsoCurrencySymbol = IsoCurrencySymbol
s.CurrencySymbol = CurrencySymbol
s.Amount = Amount
_Currencies.Add(s)
End If
End Sub
Public Sub Add(CurrencyID As Integer, IsoCurrencySymbol As String, CurrencySymbol As String, Amount As Decimal)
Dim ct As CurrencyType = _Currencies.Find(Function(obj) obj.CurrencyID = CurrencyID)
If ct IsNot Nothing Then
ct.Amount += Amount
Else
AddStructToList(CurrencyID, IsoCurrencySymbol, CurrencySymbol, Amount)
End If
End Sub
Public Sub Clear()
_Currencies.Clear()
End Sub
Public Function Count() As Integer
Return _Currencies.Count
End Function
Public Function RenderTotals() As String
' ...
End Function
End Class

Answer 1

No, you cannot do that. The whole point of inheritance it to ensure that all derived classes, if nothing else, at least share the same public interface as their base class. If you are removing a property, then it doesn't share the same interface and is therefore incompatible and not a candidate for inheritance.
If you can't say that the derived class is a type of the base class, then you can't use inheritance. For instance, I can say that an automobile is a type of vehicle, therefore, if I had an automobile class, I could have it inherit from a vehicle class. I can't however say that an insect is a type of vehicle. Therefore, even if they share most things in common, I can't have an insect class inherit from a vehicle class.
The reason for this limitation is because inheritance allows you to treat an object as if it were the base type (via type casting). For instance:
Public Sub AddPassengerToVehicle(v As Vehicle)
v.Passengers.Add(New Passenger())
End Sub
' ...
Dim auto As New Automobile()
Dim bug As New Insect()
AddPassengerToVehicle(auto) ' Works because an automobile is a type vehicle (inherits from vehicle)
AddPassengerToVehicle(bug) ' Can't possibly work (nor should it)
So, if you are in a situation where you need to have a derived class that removes/hides one of the members of its base class, you are headed in the wrong direction. In a case like that, you would need to create a whole new class which just happens to have a very similar interface, but has no direct relationship with the first class, for instance:
Public Class Vehicle
Public Property Passengers As List(Of Passenger)
Public Property MaxSpeed As Integer
Public Function SpeedIsTooFast(speed) As Boolean
Return (speed > MaxSpeed)
End Function
End Class
Public Class Insect
Public Property MaxSpeed As Integer
Public Function SpeedIsTooFast(speed) As Boolean
Return (speed > MaxSpeed)
End Function
End Class
If you want to share functionality, such as the logic in the SpeedIsTooFast method in the above example, then there are a couple different ways to do that. This first would be to make wrapper methods which simply make calls to the other class, for instance:
Public Class Insect
Private _vehicle As New Vehicle()
Public Property MaxSpeed() As Integer
Get
Return _vehicle.MaxSpeed
End Get
Set(value As Integer)
_vehicle.MaxSpeed = value
End Set
End Property
Public Function SpeedIsTooFast(speed) As Boolean
Return _vehicle.SpeedIsTooFast(speed)
End Function
End Class
If you do it this way, it would be best to have both classes implement the same common interface so that you can use them interchangeably when necessary, for instance:
Public Interface ISelfPoweredMovingThing
Property MaxSpeed As Integer
Function SpeedIsTooFast(speed As Integer) As Boolean
End Interface
Another option would be to break out the common functionality into a third class and then use that class as the base for the other two, for instance:
Public Class SelfPoweredMovingThing
Public Property MaxSpeed As Integer
Public Function SpeedIsTooFast(speed) As Boolean
Return (speed > MaxSpeed)
End Function
End Class
Public Class Vehicle
Inherits SelfPoweredMovingThing
Public Property Passengers As List(Of Passenger)
End Class
Public Class Insect
Inherits SelfPoweredMovingThing
' Anything else specific only to insects...
End Class