Try to read text , put on list , then compare on list and finally replace txt file - vb.net

Question

Trying to read a .txt file , put items to list, then on textbox change compare if the string exists in the list. Finally write the new list on the same .txt file.
Public Class Form1
Dim stockList As New List(Of String)
private sub
ListBox1.Items.AddRange(IO.File.ReadAllLines("C:\Users\path\file.txt"))
end sub
Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles
TextBox1.ReadOnlyChanged
Dim text As String = TextBox1.Text
If TextBox1.Text <> "TRAINING" Then
For Each item As Object In ListBox1.Items
If item.ToString = text Then
MsgBox("This code has already been used.", 0, "cheat attempt violation") ' Else alert the user that item already exist
Else
ListBox1.Items.Add(TextBox1.Text)
End If
Next
End If
IO.File.WriteAllLines("C:\Users\path\file.txt", ListBox1.Items.Cast(Of String).ToArray)
End Sub

Answer 1

Instead of using a UI control to store data, you should store the data in a variable. I'm not sure if you really need to show the data in a ListBox, so in the following example code I didn't.
If you use a List(Of String) instead of an array of strings, it is simpler to add another item before saving it.
The Contains method I used in the example can take a second parameter which does the comparison of the item - I guessed that you might want to ignore the case (e.g. "ABC" is the same as "aBc") so I used StringComparer.CurrentCultureIgnoreCase.
I suspect that you want to use the Control.Validating Event instead of the TextChanged event. When the data has been validated, the Control.Validated event handler is used to save it.
I put one TextBox and one Button on the form, so that the focus could change away from the TextBox, e.g. when pressing tab, to fire the validating event.
Imports System.IO
Public Class Form1
Dim dataFile As String = "C:\temp\grains.txt"
Dim alreadyUsed As List(Of String) = Nothing
Sub LoadAlreadyUsed(filename As String)
'TODO: Add exception checking, e.g., the file might not exist.
alreadyUsed = File.ReadAllLines(filename).ToList()
End Sub
Sub SaveAlreadyUsed(filename As String)
File.WriteAllLines(dataFile, alreadyUsed)
End Sub
Function CodeIsAlreadyUsed(newCode As String) As Boolean
Return alreadyUsed.Contains(newCode, StringComparer.CurrentCultureIgnoreCase)
End Function
Private Sub TextBox1_Validating(sender As Object, e As System.ComponentModel.CancelEventArgs) Handles TextBox1.Validating
' Do nothing if the user has clicked the form close button
' See https://stackoverflow.com/questions/15920770/closing-the-c-sharp-windows-form-by-avoiding-textbox-validation
If Me.ActiveControl.Equals(sender) Then
Exit Sub
End If
Dim txt = DirectCast(sender, TextBox).Text
If CodeIsAlreadyUsed(txt) Then
MessageBox.Show("This code has already been used.", "Cheat attempt violation", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
e.Cancel = True
End If
End Sub
Private Sub TextBox1_Validated(sender As Object, e As EventArgs) Handles TextBox1.Validated
' The Validated event is raised before the FormClosing event.
' We do not want to save the data when the form is closing.
If Me.ActiveControl.Equals(sender) Then
Exit Sub
End If
Dim txt = DirectCast(sender, TextBox).Text
alreadyUsed.Add(txt)
SaveAlreadyUsed(dataFile)
End Sub
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
LoadAlreadyUsed(dataFile)
End Sub
End Class