Magento module Fatal error: Class 'Mage_Cashondelivery_Helper_Data' not found in \xampp\htdocs\magento\app\Mage.php on line 516

Question

I am developing a payment Module for Magento. I did this using this tutorial

http://www.magentocommerce.com/wiki/5_-_modules_and_development/payment/create-payment-m

Answer 1

The reason your module needs a helper class is the module argument in the system.xml file:

<cashondelivery translate="label" module="cashondelivery">

Magento passes the module argument to the Mage::helper() factory method. This in turn completes the class id to cashondelivery/data.

Mage::helper('cashondelivery');
// identical to Mage::helper('cashondelivery/data');

Now Magento checks for the class prefix to use by looking for the node global/helpers/cashondelivery/class which is missing.

Rather straightforward so far, but here is where it might be a little confusing for some.
If Magento doesn't find a helper class prefix, it tries to makes one up by prefixing the class id with mage_ and appending _helper.
So this gives you mage_cashondelivery_helper as the class prefix, and mage_cashondelivery_helper_data as the full class name.

The autoloader turns this into Mage/Cashondelivery/Helper/Data.php, which can't be found, and hence the exception you are experiencing.

Besides creating the Companyname_Cashondelivery_Helper_Data class, you need to add the class group mapping to your config.xml file as follows:

<global>
  <helpers>
    <cashondelivery>
      <class>Companyname_Cashondelivery_Helper</class>
    </cashondelivery>
  </helpers>
</global>

This class group to class prefix mapping is all you are missing.