Why core/session is not setting inside __constructor in Magento?

Question

class Mymodule_Product_Block_Adminhtml_Product extends Mage_Adminhtml_Block_Widget_Grid_Container {

    public function __construct() {
        $this->_controller = "adminhtml_product";
        $this->_blockGroup = "product";
        $this->_headerText = Mage::helper("product")->__("Product Manager");
        $this->_addButtonLabel = Mage::helper("product")->__("Add New Product");
        $form_key = 123;
        Mage::getSingleton('core/session')->setFormKeyCustom($form_key);
        $this->_addButton('import', array(
            'label' => Mage::helper('product')->__('Import'),
            'onclick' => "uploadDoc('" . $this->getBaseUrl() . "product/csvupload/index/key/" . $form_key . "');",
            'class' => 'add',
            'after_html' => '',
        ));
        parent::__construct();
    }

}

Why I am not able to get that session in a frontcontroller action ?? I am getting blank Mage::getSingleton('core/session')->getFormKeyCustom();

Answer 1

You are not able to use the customer/session or core/session inside the construct of the any controller because the preDispatch method of Mage_Core_Controller_Varien_Action starts the session. The line that do that is the following

$session = Mage::getSingleton('core/session', array('name' => $this->_sessionNamespace))->start();

So, as the core/session is a singleton object if you call any method of the session object, you get a empty session forever.