VBA collection: list of keys

After I add some values to the VBA collection, is there any way to retain the list of all keys?

For example

Dim coll as new  Collection
Dim str1, str2, str3
str1="first string"
str2="second string"
str3="third string"
coll.add str1, "first key"
coll.add str2, "second key"
coll.add str3, "third key"

I know how to retain the list of strings:

first string
second string
third string

Once again: is there any way to retain the keys?

first key
second key
third key

Note: I'm using VBA through AutoCAD 2007

Solution 1:

If you intend to use the default VB6 Collection, then the easiest you can do is:

col1.add array("first key", "first string"), "first key"
col1.add array("second key", "second string"), "second key"
col1.add array("third key", "third string"), "third key"

Then you can list all values:

Dim i As Variant

For Each i In col1
  Debug.Print i(1)
Next

Or all keys:

Dim i As Variant

For Each i In col1
  Debug.Print i(0)
Next

Solution 2:

I don't thinks that possible with a vanilla collection without storing the key values in an independent array.

The easiest alternative to do this is to add a reference to the Microsoft Scripting Runtime & use a more capable Dictionary instead:

Dim dict As Dictionary
Set dict = New Dictionary

dict.Add "key1", "value1"
dict.Add "key2", "value2"

Dim key As Variant
For Each key In dict.Keys
    Debug.Print "Key: " & key, "Value: " & dict.Item(key)
Next

Solution 3:

You can create a small class to hold the key and value, and then store objects of that class in the collection.

Class KeyValue:

Public key As String
Public value As String
Public Sub Init(k As String, v As String)
    key = k
    value = v
End Sub

Then to use it:

Public Sub Test()
    Dim col As Collection, kv As KeyValue
    Set col = New Collection
    Store col, "first key", "first string"
    Store col, "second key", "second string"
    Store col, "third key", "third string"
    For Each kv In col
        Debug.Print kv.key, kv.value
    Next kv
End Sub

Private Sub Store(col As Collection, k As String, v As String)
    If (Contains(col, k)) Then
        Set kv = col(k)
        kv.value = v
    Else
        Set kv = New KeyValue
        kv.Init k, v
        col.Add kv, k
    End If
End Sub

Private Function Contains(col As Collection, key As String) As Boolean
    On Error GoTo NotFound
    Dim itm As Object
    Set itm = col(key)
    Contains = True
MyExit:
    Exit Function
NotFound:
    Contains = False
    Resume MyExit
End Function

This is of course similar to the Dictionary suggestion, except without any external dependencies. The class can be made more complex as needed if you want to store more information.

Solution 4:

An alternative solution is to store the keys in a separate Collection:

'Initialise these somewhere.
Dim Keys As Collection, Values As Collection

'Add types for K and V as necessary.
Sub Add(K, V) 
Keys.Add K
Values.Add V, K
End Sub

You can maintain a separate sort order for the keys and the values, which can be useful sometimes.