Fatal Error on Controller Accessing Session

Question

I am trying to access the session so I can redirect if the user is not logged in. Used what should be correct, from several sources:

protected $_session;

 public function __construct(\Magento\Framework\App\Action\Context $context,
                            \Magento\Framework\View\Result\PageFactory $resultPageFactory,
                            \Magento\Backend\Model\Auth\Session $authSession)
{
    parent::__construct($context);
    $this->resultPageFactory = $resultPageFactory;
    $this->_session = $authSession;
}

However I get the message on page load:

Fatal error: Uncaught TypeError: Argument 3 passed to

\Controller\Index\Index::__construct() must be an instance of Magento\Backend\Model\Auth\Session, none given, called in ..\wwwroot\var\generation....\Controller\Index\Index\Interceptor.php on line 14 and defined in ..\wwwroot\app\code....\Controller\Index\Index.php on line 15

Why is this? Have I missed something obvious (as far as I can work out this works fine on a block I have - but not on this controller and I can't see any difference.

Edit: full code Differs slightly from above because I've tried a couple of things since.

<?php
namespace ..\Subscription\Controller\Index;

use \Magento\Framework\App\Action\Action;

class Index extends Action implements \Magento\Framework\DataObject\IdentityInterface
{
    /** @var  \Magento\Framework\View\Result\Page */
    protected $resultPageFactory;
    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Magento\Customer\Model\Session $sessionSession
     */
    public function __construct(\Magento\Framework\App\Action\Context $context,
                                \Magento\Framework\View\Result\PageFactory $resultPageFactory,
                                \Magento\Customer\Model\Session $sessionSession)
    {
        parent::__construct($context);
        $this->resultPageFactory = $resultPageFactory;
        $this->_customerSession = $sessionSession;
    }

    /**
     * Subscription Index, shows overview of their subscriptions.
     *
     * @return \Magento\Framework\View\Result\PageFactory
     */
    public function execute()
    {

        // Check the user is logged in
        if($this->_customerSession->isLoggedIn()){
            return $this->resultPageFactory->create();
        }
        else {
            return $this->_redirect('customer/account/login');
        }
    }

    /**
     * Return identifiers for produced content
     *
     * @return array
     */
    public function getIdentities()
    {
        return [\..\Subscription\Model\Post::CACHE_TAG . '_' . 'list'];
    }

}

Answer 1

Delete all file from

var/generation/

And

Clear all cache.