[Resolved] How To Retrieve Keys from Collection?

[Pradeep1210]

If I know the ordinal position of an element in a collection, how can I retrieve the key associated with it?

I want to get the MyCollection.Items(2).Key ... What to do?

Is this possible in VB or with the help of some API?

[David.Poundall]

You can do this if you use the Dictionary Class. It is available if you have the references loaded for FSO.

Dictionarys work slightly quicker than Collections and they have methods for exporting the items and the keys into one dimentional arrays.

[moinkhan]

If I know the ordinal position of an element in a collection, how can I retrieve the key associated with it?

I want to get the MyCollection.Items(2).Key ... What to do?

Is this possible in VB or with the help of some API?

Default Collection object doesn't have one..
you can create your own colleciton class for this purpose..

[chemicalNova]

Or you could use something like this:

Private Declare Sub MoveMemory Lib "kernel32" Alias "RtlMoveMemory" (A As _
Any, b As Any, ByVal C As Long)
 
 
Private Declare Function lstrlenW Lib "kernel32" (ByVal lpString As Any) As _
Long
 
Public Type CollectionData
   Unk0(0 To 2) As Long
   Unk1 As Long
   ElementCount As Long
   Unk2 As Long
   PtrFirstElement As Long
   PtrLastElement As Long
   Unk3 As Long
   Unk4 As Long
   Unk5 As Long
End Type
 
 
Public Type CollectionElement
   Data As Variant
   Key As Long
   PtrPrev As Long
   PtrNext As Long
   Unk0 As Long
   Unk1 As Long
   Unk2 As Long
End Type
 
Function CollectionGetKeys(ByVal Col As Collection) As String()
Dim asKeys() As String
Dim lIdx As Long
Dim tColl As CollectionData
Dim tColElem As CollectionElement
If Col.Count > 0 Then
    ReDim asKeys(1 To Col.Count)
    MoveMemory tColl, ByVal ObjPtr(Col), LenB(tColl)
    tColElem.PtrNext = tColl.PtrFirstElement
    For lIdx = 1 To tColl.ElementCount
        MoveMemory tColElem, ByVal tColElem.PtrNext, LenB(tColElem)
        asKeys(lIdx) = pvPtr2Str(tColElem.Key)
    Next
End If
CollectionGetKeys = asKeys
End Function
 
Function pvPtr2Str(lpszA As Long) As String
pvPtr2Str = String$(lstrlenW(lpszA), 0)
MoveMemory ByVal StrPtr(pvPtr2Str), ByVal lpszA, lstrlenW(lpszA) * 2
End Function

..it can cause problems though. I'm not sure I've done it correctly. But it will return the keys you want.

chem

[moinkhan]

Or you could use something like this:

Private Declare Sub MoveMemory Lib "kernel32" Alias "RtlMoveMemory" (A As _
Any, b As Any, ByVal C As Long)
 
 
Private Declare Function lstrlenW Lib "kernel32" (ByVal lpString As Any) As _
Long
 
Public Type CollectionData
   Unk0(0 To 2) As Long
   Unk1 As Long
   ElementCount As Long
   Unk2 As Long
   PtrFirstElement As Long
   PtrLastElement As Long
   Unk3 As Long
   Unk4 As Long
   Unk5 As Long
End Type
 
 
Public Type CollectionElement
   Data As Variant
   Key As Long
   PtrPrev As Long
   PtrNext As Long
   Unk0 As Long
   Unk1 As Long
   Unk2 As Long
End Type
 
Function CollectionGetKeys(ByVal Col As Collection) As String()
Dim asKeys() As String
Dim lIdx As Long
Dim tColl As CollectionData
Dim tColElem As CollectionElement
If Col.Count > 0 Then
    ReDim asKeys(1 To Col.Count)
    MoveMemory tColl, ByVal ObjPtr(Col), LenB(tColl)
    tColElem.PtrNext = tColl.PtrFirstElement
    For lIdx = 1 To tColl.ElementCount
        MoveMemory tColElem, ByVal tColElem.PtrNext, LenB(tColElem)
        asKeys(lIdx) = pvPtr2Str(tColElem.Key)
    Next
End If
CollectionGetKeys = asKeys
End Function
 
Function pvPtr2Str(lpszA As Long) As String
pvPtr2Str = String$(lstrlenW(lpszA), 0)
MoveMemory ByVal StrPtr(pvPtr2Str), ByVal lpszA, lstrlenW(lpszA) * 2
End Function

..it can cause problems though. I'm not sure I've done it correctly. But it will return the keys you want.

chem

Chem.. y would one want to use this way as there are a lot of easier options (like creating custom class or using Dictionary object)

[chemicalNova]

When I needed it, I didn't want to create a custom class, as I only needed it for a short time to check something in my code. I was just posting this so he could use it, should the need arise.

chem

[Pradeep1210]

Yes I should use the Dictionary Object. I think I m using a Collection Object wrongly in this situation.
But as it will require major changes inmy program, I can't switch over.
I have just built my own collection class by now (as moinkhan advised)

Thanks Everyone

Here is the code for needy people like me:
This is just the general method I followed. You may wrap it into a class.

Dim MyCollection As New Collection
Dim sCollectionData(2)
 
''To add elements
sCollectionData(1) = sKey
sCollectionData(2) = sValue
MyCollection.Add sCollectionData, sKey
 
''To retrieve it
       'Retrieve by ordinal
sKey = MyCollection(AnyIndex)(1)     ''Key retrieved
sValue = MyCollection(AnyIndex)(2)   ''value retrieved
 
       'retrieve by key
sKey = MyCollection(AnyKey)(1)     ''Key retrieved
sValue = MyCollection(AnyKey)(2)   ''value retrieved

[stentor]

Public Type CollectionElement

'Data As Variant <--- don't use this

Data(0 to 15) <---- use this

Key As Long

PtrPrev As Long

PtrNext As Long

'these can be omitted since you are just reading the linked list

Unk0 As Long

Unk1 As Long

Unk2 As Long

End Type


VB initializes the variant with the automation functions for variant management. Once you write over the struct when traversing the collection's linked list, a leak at best or a crash is probable when VB calls VariantClear.

[Pradeep1210]

Do you know that you are reviving a 6 year old thread, and that too which is marked "Resolved"?

And this means bugging everyone who has participated in this thread...

[stentor]

Then quit listening to this (erronously marked as resolved) thread after 6 years already, sir. Do you need help finding the off switch? I could start a how-to thread on that if anyone is interested.

[Pradeep1210]

Nothing is marked erroneously. I was the thread starter and after coming up to a good solution for my problem, I marked it resolved.

And that was 6 years ago. Now there is no problem and no solution. Organization changed, language changed, everything changed. In fact VB6 became obsolete.

If you are in a similar problem and anything already posted in this thread helps, that's good.
Otherwise you can start your own new thread for that and many people will reply there. Why spoil a thread unnecessarily like this which contains the solution to a problem in specific situation?