Beginner programmer on sending error messages
one way to do it is to use coalesce : 

/// &lt;summary&gt;
/// Handles the Ouverture event of the PrescriptionVue control.
/// &lt;/summary&gt;
/// &lt;param name="sender"&gt;The source of the event.&lt;/param&gt;
/// &lt;param name="e"&gt;The &lt;see cref="System.EventArgs"/&gt; instance containing the event data.&lt;/param&gt;
private void PrescriptionVueOuverture(object sender, EventArgs e)
{       
    var message = ServiceCouchePolygoneFacade.Instance.RecalculerSuperficiePolygone() ?? ServicePrescriptionFacade.Instance.MettreAJourTablePrescription();

    if(message != null)
    {
        this.prescriptionVue.AfficherMessage(message);

        this.prescriptionVue.Fermer();
    }

    this.InitialiserVue();
}


the coalesce would evaluate the left-hand operand, if it's null, then it'll get the right-hand operand.