Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, _
      ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        If Not Me.ListBox1.SelectedItem Is Nothing AndAlso _
               Me.ListBox1.SelectedItem.GetType() Is GetType(MyFile) Then
            Dim f As MyFile = CType(Me.ListBox1.SelectedItem, MyFile)
            Dim fullFileName As String = f.File '// Ist der kmpl. Pfad!
            MessageBox.Show("Dateeindung ist: " + New IO.FileInfo( _
              fullFileName).Extension)
        End If
    End Sub