Public Class Form1
   
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If chiffre1.Text = "" Or op.Text = "" Or chiffre2.Text = "" Then
            MsgBox("Un champ obligatoire doit etre renseigné")
            Exit Sub
        Else
            resultat.Text = chiffre1.Text & op.Text & chiffre2.Text
        End If
    End Sub

End Class