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