public static String Evaluate(MercatorUi.MovableControls.MovableExpression currentExpression)
{
    // enter your customized code here
    MercatorUi.Forms.Billing.BillingForm billingForm = (MercatorUi.Forms.Billing.BillingForm)currentExpression.Form;
    if (billingForm.BillingEngine.CLI == null)
    {
        currentExpression.ButtonCustom.Enabled = false;
        return "0.00";
    }
    else
    {
        currentExpression.ButtonCustom.Enabled = true;
        return string.Format("{0:### ##0.00}", billingForm.BillingEngine.CliRecord.C_ENCOURS);
    }
}