VB.net basic error with String <-> Char Conversion - vb.net

Question

I was programming a little game to pratice myself and i went on an error I weren't able to fix :S. This is the start of an hanging game. (Dont know if its the correct name in english :) ) I need to take a word from a file, 1 per line, and let the player guess the word with a limited numbers of try.
I think my error is related to string/char comparaison and manipulation or, what i write in the text label. I tryed to find some tutorial or problem already solved on internet but nothing was really the same as this ... :(
I change the type of variable many time, read the debugger line per line but I never finded what was wrong .. :S If you are good with VB, plz help me fix that :O (you can also give your comment/improve)
Thx, TheFlame
Code:
Imports System.IO
Public Class Pendu
Public Structure StructMot
Public MotSecret() As Char
Public LettreDecouverte() As Char
End Structure
Dim Mot As StructMot
Dim i As Integer = 0
Private Sub ButtonA_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonA.Click, ButtonB.Click,
ButtonC.Click, ButtonD.Click, ButtonE.Click, ButtonF.Click, ButtonG.Click, ButtonH.Click, ButtonI.Click, ButtonJ.Click,
ButtonK.Click, ButtonL.Click, ButtonM.Click, ButtonN.Click, ButtonO.Click, ButtonP.Click, ButtonQ.Click, ButtonR.Click,
ButtonS.Click, ButtonT.Click, ButtonU.Click, ButtonV.Click, ButtonW.Click, ButtonX.Click, ButtonY.Click, ButtonZ.Click
i = i + 1
ActiveControl.Visible = False
PictureBox1.Image = ImageList1.Images(i - 1)
Dim j As Integer = 0
For j = 0 To Mot.MotSecret.Length - 1
If ActiveControl.Text = Mot.MotSecret(j) Then
Mot.LettreDecouverte(j) = Mot.MotSecret(j)
End If
Next j
Label1.Text = ""
For j = 0 To Mot.MotSecret.Length - 1
Label1.Text = Label1.Text + " "
If Mot.LettreDecouverte(j).Equals("") Then
Label1.Text = Label1.Text + "_"
Else
Label1.Text = Label1.Text + Mot.LettreDecouverte(j)
End If
Next j
End Sub
Private Sub JouerToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles JouerToolStripMenuItem.Click
GenereMot()
End Sub
Private Sub Pendu_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
GenereMot()
End Sub
Function GenereMot()
Dim NbItems As Integer
Dim Aleatoire As New Random()
Dim NbAleatoire As Integer
ListBox1.Items.AddRange(System.IO.File.ReadAllLines("listemot.txt"))
NbItems = ListBox1.Items.Count
NbAleatoire = Aleatoire.Next(NbItems)
Mot.MotSecret = ListBox1.Items(NbAleatoire)
Return Mot
End Function
End Class

Answer 1

the Mot.LettreDecouverte can be Nothing i.e. not initialized as you don't set any value for it.
This can cause error to happen both in Mot.LettreDecouverte(j) = Mot.MotSecret(j) line and also in If Mot.LettreDecouverte(j).Equals("") Then... line of your code.
change your ButtonA_Click event handler code as following:
i = i + 1
ActiveControl.Visible = False
PictureBox1.Image = ImageList1.Images(i - 1)
Dim j As Integer = 0
If Mot.LettreDecouverte Is Nothing OrElse Mot.LettreDecouverte.Length < Mot.MotSecret.Length Then
Mot.LettreDecouverte = Space(Mot.MotSecret.Length) '* initiate it by enough number of space chars
End If
For j = 0 To Mot.MotSecret.Length - 1
If ActiveControl.Text = Mot.MotSecret(j) Then
Mot.LettreDecouverte(j) = Mot.MotSecret(j)
End If
Next j
Label1.Text = ""
For j = 0 To Mot.MotSecret.Length - 1
Label1.Text = Label1.Text + " "
If Mot.LettreDecouverte(j).Equals(" "c) Then '*Note: i use space char, not empty string
Label1.Text = Label1.Text + "_"
Else
Label1.Text = Label1.Text + Mot.LettreDecouverte(j)
End If
Next j
Note that the value of Mot.LettreDecouverte(j) is always a char and a char can not be an empty string as you typed in Mot.LettreDeciouverte(j).Equals(""). i initiate the LettreDeciouverte with an array of space chars such that we can check its elements comparing against " "c char.