SearchOption.AllDirectories not working

Question

I'm a beginner and I'm trying to step through all files in a directory and its subdirectorys. I have this:

Imports System
Imports System.IO

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
   For Each filePath As String In IO.Directory.GetFiles("C:\test", "*.*", IO.SearchOption.AllDirectories) '
      MsgBox(filePath)
   Next filePath
End Sub

But I get the error message 'SearchOption' is not a member of 'IO'

What to do?