VB.NET structure for VB6's integer AND key indexable Dictionary?

I'm on VB.NET 2005 so I have access to generics, but I seem to keep running into trouble when trying to "translate" the VB6 Dictionary to a .NET equivalent.

It all began when translating VB6 "Dictionary" to the .NET:
Dim Pairs As Hashtable

That looked great, until I found out the code relied on the VB6 Dictionary being sorted by the item insertion order, as the indices are later matched against another table. The .NET Hashtable doesn't seem to guarantee such a sort order.

So then I tried with this:
Dim Pairs As List(Of KeyValuePair(String, String))

Great, now I had my key/value pairs in a list that should clearly be in the insertion order (latest item added last). BUT, now I can only retrieve values by their index by the place via an integer, and not via the key name. I checked with Pairs.Find(), but it of course only allows searching for the entire KeyValuePair object, not the key alone, and get the value by it.

So what I'd need is a data structure that fulfills the VB6 Dictionary behavior as in:
- Values can be retrived by integer index
- Values can be retreived by key
- The order must equal the insertion order of the items

Hmm...

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VB.NET structure for VB6's integer AND key indexable Dictionary?

Yusufp
Unfortunately, I don't think that any of the collections provided by the .NET framework meet your requirements. There is a generic Dictionary class, but like Hashtable, the ordering of its items is undefined (for purposes of enumeration). I think you have 2 options using .NET:

1) You can sort your tables before you compare them.
2) It's not totally trivial, but you can implement your own 'OrderedDictionary' as below. (NOTE, I just hacked this together as an example, and it might have bugs :)

Public Class OrderedDictionary(Of TKey, TValue)
    Implements IDictionary(Of TKey, TValue)

    Private m_dictionary As New Dictionary(Of TKey, TValue)
    Private m_keysInOrder As New List(Of TKey)

    Default Public Overloads Property Item(ByVal index As Integer) As TValue
        Get
            Return CType(m_dictionary, IDictionary(Of TKey, TValue)).Item(m_keysInOrder(index))
        End Get
        Set(ByVal value As TValue)
            CType(m_dictionary, IDictionary(Of TKey, TValue)).Item(m_keysInOrder(index)) = value
        End Set
    End Property

    Default Public Overloads Property Item(ByVal key As TKey) As TValue Implements IDictionary(Of TKey, TValue).Item
        Get
            Return CType(m_dictionary, IDictionary(Of TKey, TValue)).Item(key)
        End Get
        Set(ByVal value As TValue)
            CType(m_dictionary, IDictionary(Of TKey, TValue)).Item(key) = value
        End Set
    End Property

    Public Sub Clear() Implements ICollection(Of KeyValuePair(Of TKey, TValue)).Clear
        CType(m_dictionary, ICollection(Of KeyValuePair(Of TKey, TValue))).Clear()
        m_keysInOrder.Clear()
    End Sub

    Public Function Contains(ByVal item As KeyValuePair(Of TKey, TValue)) As Boolean Implements ICollection(Of KeyValuePair(Of TKey, TValue)).Contains
        CType(m_dictionary, ICollection(Of KeyValuePair(Of TKey, TValue))).Contains(item)
    End Function

    Public Sub CopyTo(ByVal array() As KeyValuePair(Of TKey, TValue), ByVal arrayIndex As Integer) Implements ICollection(Of KeyValuePair(Of TKey, TValue)).CopyTo
        CType(m_dictionary, ICollection(Of KeyValuePair(Of TKey, TValue))).CopyTo(array, arrayIndex)
    End Sub

    Public ReadOnly Property Count() As Integer Implements ICollection(Of KeyValuePair(Of TKey, TValue)).Count
        Get
            Return CType(m_dictionary, ICollection(Of KeyValuePair(Of TKey, TValue))).Count
        End Get
    End Property

    Public ReadOnly Property IsReadOnly() As Boolean Implements ICollection(Of KeyValuePair(Of TKey, TValue)).IsReadOnly
        Get
            Return CType(m_dictionary, ICollection(Of KeyValuePair(Of TKey, TValue))).IsReadOnly
        End Get
    End Property

    Public Function Remove(ByVal key As TKey) As Boolean Implements IDictionary(Of TKey, TValue).Remove
        Return CType(m_dictionary, IDictionary(Of TKey, TValue)).Remove(key)
        m_keysInOrder.Remove(key)
    End Function

    Public Function Remove(ByVal item As KeyValuePair(Of TKey, TValue)) As Boolean Implements ICollection(Of KeyValuePair(Of TKey, TValue)).Remove
        CType(m_dictionary, ICollection(Of KeyValuePair(Of TKey, TValue))).Remove(item)
        m_keysInOrder.Remove(item.Key)
    End Function

    Public Sub Add(ByVal key As TKey, ByVal value As TValue) Implements IDictionary(Of TKey, TValue).Add
        CType(m_dictionary, IDictionary(Of TKey, TValue)).Add(key, value)
        m_keysInOrder.Add(key)
    End Sub

    Public Sub Add(ByVal item As KeyValuePair(Of TKey, TValue)) Implements ICollection(Of KeyValuePair(Of TKey, TValue)).Add
        CType(m_dictionary, ICollection(Of KeyValuePair(Of TKey, TValue))).Add(item)
        m_keysInOrder.Add(item.Key)
    End Sub

    Public Function ContainsKey(ByVal key As TKey) As Boolean Implements IDictionary(Of TKey, TValue).ContainsKey
        CType(m_dictionary, IDictionary(Of TKey, TValue)).ContainsKey(key)
    End Function

    Public ReadOnly Property Keys() As ICollection(Of TKey) Implements IDictionary(Of TKey, TValue).Keys
        Get
            Return m_keysInOrder
        End Get
    End Property

    Public Function TryGetValue(ByVal key As TKey, ByRef value As TValue) As Boolean Implements IDictionary(Of TKey, TValue).TryGetValue
        Return CType(m_dictionary, IDictionary(Of TKey, TValue)).TryGetValue(key, value)
    End Function

    Public ReadOnly Property Values() As ICollection(Of TValue) Implements IDictionary(Of TKey, TValue).Values
        Get
            Dim rtnVal As New List(Of TValue)

            For Each key As TKey In m_keysInOrder
                rtnVal.Add(m_dictionary(key))
            Next

            Return rtnVal
        End Get
    End Property

    Public Function GetEnumerator() As IEnumerator(Of KeyValuePair(Of TKey, TValue)) Implements IEnumerable(Of KeyValuePair(Of TKey, TValue)).GetEnumerator
        Return New Enumerator(Me)
    End Function

    Private Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator
        Return New Enumerator(Me)
    End Function

    Public Structure Enumerator
        Implements IEnumerator(Of KeyValuePair(Of TKey, TValue))

        Private m_dictionary As OrderedDictionary(Of TKey, TValue)
        Private m_nextIndex As Integer
        Private m_current As KeyValuePair(Of TKey, TValue)

        Friend Sub New(ByVal orderedDictionary As OrderedDictionary(Of TKey, TValue))
            m_dictionary = orderedDictionary
            m_nextIndex = 0
            m_current = New KeyValuePair(Of TKey, TValue)
        End Sub

        Public ReadOnly Property Current() As KeyValuePair(Of TKey, TValue) Implements IEnumerator(Of KeyValuePair(Of TKey, TValue)).Current
            Get
                Return New KeyValuePair(Of TKey, TValue)(m_current.Key, m_current.Value)
            End Get
        End Property

        Private ReadOnly Property Current1() As Object Implements IEnumerator.Current
            Get
                Return Current
            End Get
        End Property

        Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext
            Dim rtnVal As Boolean = False

            If m_nextIndex < m_dictionary.Count Then
                rtnVal = True
                Dim key As TKey = m_dictionary.m_keysInOrder(m_nextIndex)
                m_current = New KeyValuePair(Of TKey, TValue)(key, m_dictionary.Item(key))
                m_nextIndex += 1
            Else
                m_current = New KeyValuePair(Of TKey, TValue)
            End If

            Return rtnVal
        End Function

        Public Sub Reset() Implements System.Collections.IEnumerator.Reset
            m_nextIndex = 0
        End Sub

        Public Sub Dispose() Implements System.IDisposable.Dispose
        End Sub
    End Structure
End Class

Hope that helps,

Kevin
Talbot McInnis
Thanks for your data structure class. :)

I was afraid it would end as something like this, but at least I have another opinion on the subject now so I feel less that I might have missed something, and I can maybe work with changing the design instead, or look into your class. ;) Hm, I may actually end up simply building a List alongside a Dictionary. That kind of overhead doesn't really matter as it's done outside a loop and the memory consumptions won't be big.

I can't really blame MS for this, as Hashtables normally *don't* have well-defined orderings, and only list-style structures have for sure. The problem is more that the code exploits a number of peculiarities with the VB6 design...
VB.NET structure for VB6's integer AND key indexable Dictionary?

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