Creating a Custom Enumerator in a Base Class

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  • Charles Law
    #1

    Creating a Custom Enumerator in a Base Class

    I have a base class MyBaseClass, and several classes that inherit from it:
    MyClass1, MyClass2, etc.

    The base class implements IEnumerable(Of IMyBaseClassRow ). The base class
    has a DataTable object that contains different data depending on which of
    MyClass1, MyClass2 are instantiated.

    I want to be able to iterate through the rows of the data table using For
    Each on my derived classes retrieving a custom row with properties specific
    to that child class. I found this article

    http://blog.falafel.com/2007/01/30/CreateACustomDataSourceQuicklyEasilyWithC20Iterators.aspx


    which alludes to the technique, but doesn't go all the way to showing how to
    derive custom row classes with the properties. I am also doing this in VB,
    so I don't have Yield Return. Therefore, I am having to code this all by
    hand.

    I want all the enumeration stuff in the base class, so that I don't have to
    reproduce it in every derived class, but I have one stumbling block. This is
    my implementation of Current() in my base class:

    Private ReadOnly Property Current() As IMyBaseClassRow Implements
    System.Collecti ons.Generic.IEn umerator(Of IMyBaseClassRow ).Current
    Get
    If m_CurrentRow = -1 Or m_CurrentRow >
    m_MyBaseClass.D ataTable.Rows.C ount - 1 Then
    Throw New InvalidOperatio nException
    End If

    Return New
    MyBaseClassRow( m_MyBaseClass.D ataTable.Rows(m _CurrentRow))
    End Get
    End Property

    The problem is that this can only return a new MyBaseClassRow, and not a
    MyClass1Row. Therefore, when I do

    For Each row As MyClass1Row in MyClass1Instanc e
    ...
    Next

    the compiler is happy, but it fails at runtime with an InvalidCastExce ption
    on the For Each line.

    I think I need to override something in my MyClass1 or MyClass1Row class,
    but I'm not sure what exactly, or how.

    If this problem makes sense to anyone, please feel free to chip in. Any and
    all help welcome.

    TIA

    Charles


    Tags: None
    • Charles Law
      #2
      Re: Creating a Custom Enumerator in a Base Class

      I think I've cracked it.

      In my base class MyBaseClass, I have created a must override function that
      returns an object of the type I want, i.e.

      Protected MustOverride Function GetIMyBaseClass Row(dr As DataRow) As
      IMyBaseClassRow

      In my concrete class, I then override this function:

      Protected Overrides Function GetIMyBaseClass Row(ByVal dr As
      System.Data.Dat aRow) As IMyBaseClassRow
      Return New MyClass1Row(dr)
      End Function

      Finally, Current() in MyBaseClass becomes this:

      Private ReadOnly Property Current() As IMyBaseClassRow Implements
      System.Collecti ons.Generic.IEn umerator(Of IMyBaseClassRow ).Current
      Get
      If m_CurrentRow = -1 Or m_CurrentRow >
      m_MyBaseClass.D ataTable.Rows.C ount - 1 Then
      Throw New InvalidOperatio nException
      End If

      Return
      GetIMyBaseClass Row(m_MyBaseCla ss.DataTable.Ro ws(m_CurrentRow ))
      End Get
      End Property

      It allows the base class to call the derived class to get an object of the
      actual type required.

      I hope this helps someone.

      If anyone has another solution, I am still very interested to hear.

      Charles


      "Charles Law" <blank@nowhere. comwrote in message
      news:OWpQYelGJH A.4408@TK2MSFTN GP04.phx.gbl...
      >I have a base class MyBaseClass, and several classes that inherit from it:
      >MyClass1, MyClass2, etc.
      >
      The base class implements IEnumerable(Of IMyBaseClassRow ). The base class
      has a DataTable object that contains different data depending on which of
      MyClass1, MyClass2 are instantiated.
      >
      I want to be able to iterate through the rows of the data table using For
      Each on my derived classes retrieving a custom row with properties
      specific to that child class. I found this article
      >
      http://blog.falafel.com/2007/01/30/CreateACustomDataSourceQuicklyEasilyWithC20Iterators.aspx

      >
      which alludes to the technique, but doesn't go all the way to showing how
      to derive custom row classes with the properties. I am also doing this in
      VB, so I don't have Yield Return. Therefore, I am having to code this all
      by hand.
      >
      I want all the enumeration stuff in the base class, so that I don't have
      to reproduce it in every derived class, but I have one stumbling block.
      This is my implementation of Current() in my base class:
      >
      Private ReadOnly Property Current() As IMyBaseClassRow Implements
      System.Collecti ons.Generic.IEn umerator(Of IMyBaseClassRow ).Current
      Get
      If m_CurrentRow = -1 Or m_CurrentRow >
      m_MyBaseClass.D ataTable.Rows.C ount - 1 Then
      Throw New InvalidOperatio nException
      End If
      >
      Return New
      MyBaseClassRow( m_MyBaseClass.D ataTable.Rows(m _CurrentRow))
      End Get
      End Property
      >
      The problem is that this can only return a new MyBaseClassRow, and not a
      MyClass1Row. Therefore, when I do
      >
      For Each row As MyClass1Row in MyClass1Instanc e
      ...
      Next
      >
      the compiler is happy, but it fails at runtime with an
      InvalidCastExce ption on the For Each line.
      >
      I think I need to override something in my MyClass1 or MyClass1Row class,
      but I'm not sure what exactly, or how.
      >
      If this problem makes sense to anyone, please feel free to chip in. Any
      and all help welcome.
      >
      TIA
      >
      Charles
      >
      >

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