Private Function ConvertDate(ByVal input As String) As Date
        If String.IsNullOrEmpty(input) Then
            Return Nothing
        End If
 
        If input.Length < 6 Then
            Return Nothing
        End If
 
        Try
            Return Date.Parse(input.Substring(0, 2) & "." & input.Substring(2, _
              2) & "." & input.Substring(4, 2))
        Catch ex As Exception
            Return Nothing
        End Try
    End Function

        Dim myString As String = "231109-Char-12345"
        Dim myDate As Date = ConvertDate(myString)
 
        If Not myDate = Nothing Then
            MsgBox(myDate.ToLongDateString, MsgBoxStyle.Information)
        Else
            MsgBox("Dies ist kein gültiges Datum.", MsgBoxStyle.Critical)
        End If