Option Explicit
Public Sub OnAction_ActivateOrOpenWorkbook()
Const C_FILE As String = "zeitraum2.xlsm"
Dim wkb As Excel.Workbook
Dim strFilename As String
strFilename = IIf(Right$(ThisWorkbook.Path, 1) <> "\", ThisWorkbook.Path & "\", ThisWorkbook.Path)
strFilename = strFilename & C_FILE
If Dir$(strFilename, vbNormal) = "" Then
Call MsgBox("Datei " & strFilename & " wurde nicht gefunden!", vbCritical)
Exit Sub
End If
On Error Resume Next
Set wkb = Workbooks(C_FILE)
On Error GoTo 0
If Not wkb Is Nothing Then
Call wkb.Activate
Exit Sub
End If
Call Workbooks.Open(strFilename)
End Sub