Запретить пользователю изменять данные в файле

Imports System.IO
Imports System.Security.Cryptography
Imports System.Text
 
Class CryptoHelper
 
    Shared IV As Byte() = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16} 'Ключ
    Shared Key As Byte() = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16} 'Вектор инициализации
    Public Shared Function OpenReadStream(ByVal fileName As String) As Stream
        Using rijndael As Rijndael = RijndaelManaged.Create()
            rijndael.IV = IV
            rijndael.Key = Key
            Return New CryptoStream(File.OpenRead(fileName), rijndael.CreateDecryptor(), CryptoStreamMode.Read)
        End Using
    End Function
    Public Shared Function OpenWriteStream(ByVal fileName As String) As Stream
        Using rijndael As Rijndael = RijndaelManaged.Create()
            rijndael.IV = IV
            rijndael.Key = Key
            Return New CryptoStream(File.Create(fileName), rijndael.CreateEncryptor(), CryptoStreamMode.Write)
        End Using
    End Function
 
End Class
 
Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim txt As String = TextBox1.Text
        Using writer As New StreamWriter(CryptoHelper.OpenWriteStream("d:\01.txt"))
            writer.WriteLine(txt) 'Пишем в CryptoStream
        End Using
    End Sub
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Using reader As New StreamReader(CryptoHelper.OpenReadStream("d:\01.txt"), Encoding.GetEncoding(1251))
            Do While reader.Peek() >= 0
                Dim line As String = reader.ReadLine()
                ComboBox1.Items.Add(line)
                TextBox2.Text = line
            Loop
        End Using
    End Sub
 
End Class

TextBox2.Text = reader.ReadToEnd() ' в MessageBox же выводит
'или
TextBox2.Text = reader.ReadLine()

TextBox2.Text = line

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Using reader As New StreamReader(CryptoHelper.OpenReadStream("d:\01.txt"), Encoding.GetEncoding(1251))
            TextBox2.Text = reader.ReadToEnd()
        End Using
    End Sub

 Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Using reader As New StreamReader(CryptoHelper.OpenReadStream("d:\01.txt"), Encoding.GetEncoding(1251))
            Do While reader.Peek() >= 0
                Dim line As String = reader.ReadLine()
                ComboBox1.Items.Add(line)
            Loop
        End Using
    End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Using reader As New StreamReader(CryptoHelper.OpenReadStream("d:\01.txt"), Encoding.GetEncoding(1251))
            Do While reader.Peek() >= 0
                Dim line As String = reader.ReadLine()
                TextBox2.Text = reader.ReadToEnd()
                ComboBox1.Items.Add(line)
            Loop
        End Using
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Using reader As New StreamReader(CryptoHelper.OpenReadStream("d:\01.txt"), Encoding.GetEncoding(1251))
            Do While reader.Peek() >= 0
                Dim line As String = reader.ReadLine()
                ComboBox1.Items.Add(line)
            Loop
        End Using
        Using reader As New StreamReader(CryptoHelper.OpenReadStream("d:\01.txt"), Encoding.GetEncoding(1251))
            TextBox2.Text = (reader.ReadToEnd())
        End Using
    End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
    Dim allText As String
    Using reader As New StreamReader(CryptoHelper.OpenReadStream("d:\01.txt"), Encoding.GetEncoding(1251))
        allText = reader.ReadToEnd()
    End Using
 
    TextBox2.Text = allText
    For Each Line As String In TextBox2.Lines
        ComboBox1.Items.Add(Line)
    Next
End Sub