Private Sub Workbook_Open()
Dim T() As String, Obj As Object, L As Integer
ReDim T(1 To Me.Sheets.Count)
For Each Obj In Me.Sheets
L = L + 1: T(L) = Obj.Name
Next Obj
With Feuil1.[G5].Validation
.Delete
.Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
xlBetween, Formula1:=Join(T, ",")
.IgnoreBlank = True
.InCellDropdown = True
.InputTitle = ""
.ErrorTitle = ""
.InputMessage = ""
.ErrorMessage = ""
.ShowInput = True
.ShowError = True
End With
End Sub