Hi
Here is an example to do the same. I suggest you trace your own code to see what/why it isn't working as you expect. Are you familiar with using BreakPoints etc?
This is a stand alone example. The Form1 only has ListBox1 and TextBox1 (multiline) on it.
Option Strict On
Option Explicit On
Public Class Form1
' these variable would likely be set from user
' selecting the folder with (say) a FolderBrowser
' but just hard coded here for this example
Dim fold As String = My.Computer.FileSystem.SpecialDirectories.Desktop
Dim files() As String = IO.Directory.GetFiles(fold, "*.txt")
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
ListBox1.Items.AddRange(files)
End Sub
Private Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ListBox1.SelectedIndexChanged
Dim file As String = ListBox1.SelectedItem.ToString
TextBox1.Text = IO.File.ReadAllText(file)
End Sub
End Class