title: ArgumentOutOfRangeException VB
body: <p>The user is supposed to write is health insurance card number in the "numero" textbox. The number is 4 letters then 8 digits (LANS10763985).
  My problem is that when I left the textbox empty, my app crash!
  How can I just put a MsgBox that says to enter something!!!!
  Noob here sorry!</p>
  
  <pre><code>    Dim numero As String 'Numéro d'assurance maladie
      Dim lettre As String 'Quatre premières lettres 
      Dim chiffre1 As String 'Quatre premiers chiffres
      Dim chiffre2 As String 'Quatre derniers chiffres
  
      numero = txtNumero.Text
      lettre = txtNumero.Text.Substring(0, 4)
      chiffre1 = txtNumero.Text.Substring(4, 4)
      chiffre2 = txtNumero.Text.Substring(8, 4)
  </code></pre>