How to prevent duplicate item.

[Elroy]

Here, something like this is what I'd propose. If you further manipulate the listbox though, you'd need to do further work with the collection to make sure it stayed mirrored:

Option Explicit

Dim coll As New Collection


Private Sub Form_Load()

    Dim i As Long
    Dim s As String

    For i = 1 To 1000
        s = sRandomString(2)
        If bAddedToCollection(s) Then List1.AddItem s
    Next i


    MsgBox List1.ListCount

End Sub

Public Function bAddedToCollection(sData As String) As Boolean
    ' Returns false if already in collection.
    On Error GoTo HadError
    coll.Add sData, sData
    bAddedToCollection = True
HadError:
End Function

Public Function sRandomString(iLength As Integer) As String
    Dim i As Integer
    Dim j As Integer
    Dim s As String
    Static b As Boolean
    '
    If Not b Then
        Randomize
        b = True
    End If
    '
    Do Until j = iLength
        ' This returns an integer from 48 to 83.
        i = Int(36 * Rnd + 48)
        ' Skip over characters between 9 and A.
        If i > 57 Then i = i + 7
        ' Now, i is between 48 and 57 or 65 and 90.
        '                   "0"    "9"   "A"    "Z"
        s = s + Chr$(i)
        j = j + 1
    Loop
    sRandomString = s
End Function

EDIT1: Vb6Lovers code also looks quite interesting, and it's certainly more memory efficient than mine.

EDIT2: However, just out of curiousity, I timed both methods. The Collection method seems to be WAY faster. I suspect that this is true because the API still has to loop through the listbox, and a collection has a binary-tree index that can be searched (which is much faster than a loop).

After running it a few times, my results were:

API Seconds:  1.574219 
Collection Seconds:  0.3046875 
API Seconds:  1.546875 
Collection Seconds:  0.3007813 
API Seconds:  1.5625 
Collection Seconds:  0.296875

And here's my patched up code I used to test the timing:

Option Explicit

Dim coll As New Collection

Private Const LB_FINDSTRINGEXACT = &H1A2
Private Const LB_ADDSTRING = &H180

Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
'


Public Function bFoundInListBox(s As String, lst As ListBox) As Boolean
    Dim iRet As Long
    iRet = SendMessage(lst.hwnd, LB_FINDSTRINGEXACT, -1&, ByVal s)
    bFoundInListBox = iRet > -1&
End Function

Private Sub Form_Load()
    
    Dim i As Long
    Dim s As String
    Dim nStart As Single
    
    
    nStart = Timer
    For i = 1 To 3000
        s = sRandomString(3)
        If Not bFoundInListBox(s, List1) Then List1.AddItem s
    Next i
    Debug.Print "API Seconds: "; Timer - nStart



    
    
    nStart = Timer
    For i = 1 To 3000
        s = sRandomString(3)
        If bAddedToCollection(s) Then List2.AddItem s
    Next i
    Debug.Print "Collection Seconds: "; Timer - nStart
    
    
    MsgBox List1.ListCount
    
End Sub

Public Function bAddedToCollection(sData As String) As Boolean
    ' Returns false if already in collection.
    On Error GoTo HadError
    coll.Add sData, sData
    bAddedToCollection = True
HadError:
End Function

Public Function sRandomString(iLength As Integer) As String
    Dim i As Integer
    Dim j As Integer
    Dim s As String
    Static b As Boolean
    '
    If Not b Then
        Randomize
        b = True
    End If
    '
    Do Until j = iLength
        ' This returns an integer from 48 to 83.
        i = Int(36 * Rnd + 48)
        ' Skip over characters between 9 and A.
        If i > 57 Then i = i + 7
        ' Now, i is between 48 and 57 or 65 and 90.
        '                   "0"    "9"   "A"    "Z"
        s = s + Chr$(i)
        j = j + 1
    Loop
    sRandomString = s
End Function