Visual Basic - Ayuda con este código

Imports System.IO

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        If FolderBrowserDialog1.ShowDialog() = DialogResult.OK Then

            ' List files in the folder.

            'ListFiles(FolderBrowserDialog1.SelectedPath)

        End If

    End Sub

    Private Sub ListFiles(ByVal folderPath As String)

        filesListBox.Items.Clear()

        Dim fileNames As String() =

            System.IO.Directory.GetFiles(folderPath,

                "*.*", System.IO.SearchOption.TopDirectoryOnly)

        For Each fileName As String In fileNames

            'filesListBox.Items.Add(fileName)

            Dim theString As String = "Serial number :"

            Dim path As String = fileName

            'Dim value As String = File.ReadAllText(fileName)

            Dim array() As String = File.ReadAllLines(path)

            filesListBox.Items.Add(array)

            Dim line As String

            For Each line In array

                ' We now have the line so can use it.

                Dim length As Integer = line.Length

                Dim TestPos As Integer

                If TestPos = InStr(1, length, theString, vbTextCompare) > 0 Then

                    Dim Result = Mid(length, TestPos, 20)

                    'Debug.Print(fileName)

                    MsgBox(Result)

                End If

            Next

        Next

    End Sub

End Class