Binary Reader and allowed characters - vb.net

Question

I have a problem.
Now program working like this:
Reading binary file bytes. &H1E and &H1F Allowed characters only :A123456789
If in file is B or C or D or E or F....... textbox1 = bad file
That's working. Now I want to add verification in &H10.
Allowed characters only 26
If other characters textbox1 = bad file
Imports System.IO
Imports System.IO.SeekOrigin
Public Class Form1
Dim charactersAllowed As String = "A123456789"
Dim swap As String = "26"
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim OFD As New OpenFileDialog
Dim fullFile() As Byte
With OFD
.Filter = "Binary files (*.bin)|*.bin"
End With
If OFD.ShowDialog = Windows.Forms.DialogResult.OK Then
fullFile = File.ReadAllBytes(OFD.FileName)
TextBox1.AppendText(fullFile(&H1E).ToString("X2") & " ")
TextBox1.AppendText(fullFile(&H1F).ToString("X2"))
TextBox4.AppendText(fullFile(&H10).ToString("X2"))
End If
Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged
Dim theText As String = TextBox1.Text
Dim Letter As String
Dim SelectionIndex As Integer = TextBox1.SelectionStart
Dim Change As Integer
For x As Integer = 0 To TextBox1.Text.Length - 1
Letter = TextBox1.Text.Substring(x, 1)
If charactersAllowed.Contains(Letter) = False Then
theText = theText.Replace(Letter, String.Empty)
Change = 1
End If
Next
TextBox1.Text = theText
TextBox1.Select(SelectionIndex - Change, 0)
End Sub
Private Sub TextBox4_TextChanged(sender As Object, e As EventArgs) Handles TextBox4.TextAlignChanged
Dim check As String = TextBox4.Text
Dim check2 As String
Dim check3 As Integer = TextBox4.SelectionStart
Dim check4 As Integer
For x As Integer = 0 To TextBox4.Text.Length - 1
check2 = TextBox4.Text.Substring(x, 1)
If swap.Contains(check2) = False Then
check = check.Replace(check2, String.Empty)
check4 = 1
End If
Next
TextBox1.Text = check
TextBox1.Select(check3 - check4, 0)
End Sub

Answer 1

Hard to decode, but an obvious approach is to prevent editing when the byte has the wrong value:
If OFD.ShowDialog = Windows.Forms.DialogResult.OK Then
fullFile = File.ReadAllBytes(OFD.FileName)
If fullFile(&H10) <> 26 Then
TextBox1.Text = "Bad file"
TextBox1.Enabled = False
TextBox4.Enabled = False
Else
TextBox1.Enabled = True
TextBox4.Enabled = True
'' etc...
End If
End If