WC_Order_Item_Product::set_tax_class PHP Method

    public function set_tax_class($value)
    {
        if ($value && !in_array($value, WC_Tax::get_tax_classes())) {
            $this->error('order_item_product_invalid_tax_class', __('Invalid tax class', 'woocommerce'));
        }
        $this->set_prop('tax_class', $value);
    }