public void calcul_textboxs()
{
double tva = facture.calculTVA(Convert.ToDouble(this.textBoxTTC.Text));
double ht = facture.calculHT(Convert.ToDouble(this.textBoxTTC.Text), tva);
textBoxTVA.Text = Convert.ToString(tva);
textBoxHT.Text = Convert.ToString(ht);
}