if liste.contains(a) then 
 usw.

If not EventInListe(EventID, EventListe) then
   do something
end if

Function EventInListe(ByVal id As String, ByVal liste As String()) As Boolean
        For Each e As String In liste
            If e = id Then
                EventInListe = True
                Exit For
            End If
            EventInListe = False
        Next
End Function