Prevent Duplicate Listbox Entries

This useful code listing prevents users from entering duplicate entries into a listbox or modifying existing listbox entries. While the code prevents users from adding duplicate entries, you can modify the True condition of the If block in the Add_New procedure to suit your needs. Declare the constant, LB_FINDSTRINGEXACT and the SendMessage function declaration in a BAS module:

 Declare Function SendMessageByString Lib _	"user32" Alias "SendMessageA" (ByVal hWnd As _	Long, ByVal wMsg As Long, ByVal wParam As _	Long, ByVal lParam As String) As LongPublic Const LB_FINDSTRINGEXACT = &H1A2

This function uses the SendMessageByString API and returns either the existing item’s index or -1. If it returns -1, you can add the item:

 Function ChkListDuplicates(chwnd As Long, _	StrText As String) As Boolean	ChkListDuplicates = (SendMessageByString(chwnd, _		LB_FINDSTRINGEXACT, -1, StrText) > -1)End Function

If the LB_FINDEXACTSTRING message returns a value of -1, no match was found, so ChkListDuplicates returns False. You can use this value to determine whether to add a new item to your list:

 Private Sub Add_New()	Dim user As String	user = InputBox("Add ListBox Entry", "Test Project")	If Len(user) Then		If Not ChkListDuplicates(List1.hWnd, _			Trim(user)) Then			List1.AddItem Trim(user)		Else			MsgBox "Duplicate Names can not be " & _				"added." & vbCrLf & "Please " & _				"make sure you are not adding " & _				"duplicate names.", vbExclamation, _				"Test Project: Invalid Operation"		End If	End IfEnd Sub
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