This is a follow-up question to this one:

Excel Ribbon comboBox: How to set up a getSelectedItemIndex Callback

The answer to that revealed:

• Call back getSelectedItemIndex is available for dropDown instead of comboBox.

Question: Is there any other way to get the index of the selection in a comboBox?

This answer uses a Dictionary to build the onChange Callback.

Would this be the way to go?

comboBox:

' -- XML

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="LoadRibbon">
    <ribbon>
        <tabs>
            <tab id="Tabv3.1" label="TOOLS" insertAfterMso="TabHome">                                             
                <group id="GroupDemo2" 
                label="SelectPapersize"
                imageMso="AddInManager">
                    <comboBox id="ComboBox001"
                    label="comboBox001"
                    getText="ComboBox001_GetText"
                    onChange="ComboBox001_OnChange">
                        <item id="Item_A3"
                        label="A3"/>
                        <item id="Item_A4"
                        label="A4"/>
                        <item id="Item_A5"
                        label="A5"/>
                    </comboBox>
                </group>   
            </tab>
        </tabs>
    </ribbon>
</customUI>


' -- Callback VBA in Module "RibbonCallbacks"

Option Explicit
Public RibUI As IRibbonUI
Public Const myApp As String = "RibbApp", mySett As String = "Settings", myVal As String = "Value"

Sub LoadRibbon(Ribbon As IRibbonUI)
    Set RibUI = Ribbon
    RibUI.InvalidateControl "ComboBox001"
End Sub

'Callback for ComboBox001 onChange
Sub ComboBox001_OnChange(control As IRibbonControl, id As String)
    Select Case id
        Case "A3"
             ActiveSheet.PageSetup.PaperSize = xlPaperA3
        Case "A4"
            ActiveSheet.PageSetup.PaperSize = xlPaperA4
        Case "A5"
            ActiveSheet.PageSetup.PaperSize = xlPaperA5
    End Select
    RibUI.InvalidateControl "ComboBox001"
    SaveSetting myApp, mySett, myVal, id
End Sub

'Callback for ComboBox001 getText
Sub ComboBox001_getText(control As IRibbonControl, ByRef returnedVal)
  Dim comboVal As String
    comboVal = GetSetting(myApp, mySett, myVal, "No Value") 
    If comboVal <> "No Value" Then
      returnedVal = comboVal
    End If
End Sub


' -- VBA in "ThisWorkbook"

Private Sub Workbook_Open()

End Sub

Private Sub Workbook_SheetActivate(ByVal Sh As Object)
    Dim idx As String
    Select Case PaperSize
    Case "xlPaperA3"
        idx = "A3"
    Case "xlPaperA4"
        idx = "A4"
    Case "xlPaperA5"
        idx = "A5"
    End Select
    SaveSetting myApp, mySett, myVal, idx
    RibUI.InvalidateControl "ComboBox001"
End Sub

onChange CallBack using a Dictionary:

'/questions/60453051/ribbon-xml-get-selected-index-or-item-id-in-a-combobox-control

<customUI onLoad="Ribbon_Load" xmlns="http://schemas.microsoft.com/office/2009/07/customui">
<ribbon>
  <tabs>

    <tab id="SearchCustomerTab" insertAfterMso="TabAddIns" label="Cliente" visible="true">
      <group id="SearchCustomerGroup" label="Cliente" autoScale="true">
        <comboBox id="CustomerComboBox" getItemCount="GetItemCountCallback" getItemLabel="GetItemLabelCallback" getItemID="GetItemIDCallback" onChange="OnChangeCallback" />
      </group>
    </tab>
  </tabs>
 </ribbon>
</customUI>


rivate customClass As CustomClass
Private customDictionary As Dictionary(Of String, CustomClass)

Public Sub Ribbon_Load(ByVal ribbonUI As Office.IRibbonUI)
    Dim customList As List(Of CustomClass)

    customList = FunctionToPopulateMyList()
    customDictionary = customList.ToDictionary(Function(p) p.MyText, Function(p) p)

End Sub

Public Function GetItemLabelCallback(ByVal control As Office.IRibbonControl, index As Integer) As String

        Return oCustomDictionary.ElementAt(index).Value.MyText
End Function

Public Function GetItemCountCallback(ByVal control As Office.IRibbonControl) As Integer

        Return oCustomDictionary.Count

End Function

Public Function GetItemIDCallback(ByVal control As Office.IRibbonControl, index As Integer) As String
    Return "Item" & index.ToString & control.Id

End Function

Public Sub OnChangeCallback(ByVal control As Office.IRibbonControl, text As String)
    If (customDictionary.ContainsKey(text)) Then
        customClass = customDictionary(text)

    End If
End Function