CODEWITHSUNDEEP
.netvb.netinheritance
Adam Speight
Adam Speight

Reputation: 732

Base-Class only Methods?

Is there a way of only-base class only methods?

I have a work around of using a module, but this separates the functionality which will only be utilized by the base-class.

I was think on the line of the following

Public MustInherit Class Token
  ' Token stuff
  NotInheritable Shared Function Parse(Of T As Token)(CR As CharReader) As T
    ' Would also be good to be able to do the following without resorting
    ' to the reflection based bodgery.
    Return T.Parser(CR)
  End Function
End Class

Public Class Digit
  Inherit Token
  ' Digit Stuff
  Protected Shared Function Parser(CR As CharReader) As Digit
    If CR.Current.HasValue = False Then Return Nothing
      Case Select CR.Value
        Case "0"c To "9"c
          Return New Digit(CR.Index,0)
      Case Else
        Return False
      End Select
  End Function

So now when

Dim d0 = Token.Parse(Of Digit)(cr)

but

Dim d1 = Digit.

wouldn't show the Parse Method.

So how can this be done? (If possible at all)

EDIT

Current Implementations This should really a Base Class only method in the Token Class

Public Module TokenModule
  Public Function Parse(Of T As Token)(cr As CharReader) As T
  '  
  ' Here Be Nasty Reflection Based Bodge Job 
  '
  ' Why? What I want to write. ( Call a static method on the generic (constrianed) type     specifier.)
  '
  ' Return T.Parser(cr)
  ' 
  ' Start Bodgery {
  Dim tt As T
  tt = GetType(T).InvokeMember("Parser",
                                Reflection.BindingFlags.InvokeMethod +
                                Reflection.BindingFlags.NonPublic +
                                Reflection.BindingFlags.Static, Nothing, tt, {cr})
  Return tt
  ' } End Bodgery
End Function
End Module

Token (Base) Class

Public MustInherit Class Token
  Private _Index As Integer
  Private _Count As Integer
  Protected Friend Sub New(ByVal Index As Integer, Count As Integer)
    _Index = Index : _Count = Count
  End Sub
  Public ReadOnly Property Index As Integer
    Get
      Return _Index
    End Get
  End Property
  Public ReadOnly Property Count As Integer
    Get
      Return _Count
    End Get
  End Property
  Protected Shared Function Parser(cr As CharReader) As Token
    Return Nothing
  End Function
End Class

Digit Class

Public Class Digit
  Inherits Token.Token
  Private Sub New(ByVal Index As Integer, Count As Integer)
    MyBase.New(Index, Count)
  End Sub
  Protected Overloads Shared Function Parser(cr As CharReader) As Digit
    Dim crc = cr.Current
    If crc.HasValue = False Then Return Nothing 
      Select Case crc.Value
        Case "0"c To "9"c
          Return New Digit(cr.Index, 1)
        Case Else
          Return Nothing
        End Select
  End Function
End Class

Upvotes: 1

Views: 456

Answers (3)

Justin Niessner
Justin Niessner

Reputation: 245419

That kind of relationship really breaks the OO Concepts. If a parent type provides public functionality, then the child should as well (think Child is a Parent).

The kind of functionality you want can easily be achieved via composition (Digit would contain an instance of Token and proxy whatever calls are necessary to that instance) rather than inheritance.

Upvotes: 2

Hans Passant
Hans Passant

Reputation: 941377

wouldn't show the Parse Method.

Not sure I follow but you only seem concerned about IntelliSense showing the method. That's easy to solve with EditorBrowsableAttribute:

Imports System.ComponentModel
...
Class Token
    Public Shared Sub Parse()
        '' etc
    End Sub
End Class

Class Digit
    Inherits Token

    <EditorBrowsable(EditorBrowsableState.Never)> _
    Public Shared Shadows Sub Parse()
        Token.Parse()
    End Sub
End Class

Upvotes: 1

Olivier Jacot-Descombes
Olivier Jacot-Descombes

Reputation: 112324

If CharReader implements IConvertible, you can convert it to other types with System.Convert. The standard types like Boolean, String, Char, Int32, Double, etc. implement IConvertible. 

Shared Function Parse(Of T As Token)(ByVal CR As CharReader) As T
    Return DirectCast(Convert.ChangeType(CR, GetType(T)), T)
End Function

--

Parse can be hidden by shadowing it with a non public method

Protected Shared Shadows Function Parse(ByVal CR As CharReader) As Digit
                    ^
                    |
            (note the Shadows keyword)

Note that this works only for shared methods, since you access them through the type name and not through an instance.

Upvotes: 0

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