J'ai un trou de mémoire je me rappel plus c'est quoi le code pour que le button genere que des lettres et quand je coche la case pour les chiffres sa en genere 

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Dim s As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklnmopqrstuvwxz-" 'ce qui sera generer (les caracteres)
Dim r As New Random
Dim sb As New StringBuilder
For i As Integer = 1 To 20 '20 est le nombre de caracteres
Dim idx As Integer = r.Next(0, 35)
sb.Append(s.Substring(idx, 1))
Next
TextBox1.Text = sb.ToString()
End Sub