How to add extension attribute in Magento2

Here we learn, how to add extension attribute in magento2. Extension attributes used to extend interface functionality.

1. Create app/code/Webkul/DemoModule/etc/extension_attributes.xml

<?xml version="1.0"?>
<!-- 
	/**
 * Webkul Software.
 *
 * @category  Webkul
 * @package   Webkul_DemoModule
 * @author    Webkul
 * @copyright Copyright (c) 2010-2016 Webkul Software Private Limited (https://webkul.com)
 * @license   https://store.webkul.com/license.html
 */
 -->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\Checkout\Api\Data\ShippingInformationInterface">
        <attribute code="custom_shipping_charge" type="string"/>
    </extension_attributes>
</config>

2. Now clear the var/generation when you run setup:di:compile command, new getter and setter methods will be added in /var/generation/Magento/Checkout/Api/Data/ShippingInformationExtensionInterface.php

3. Now you can set/get these attributes value by creating the instance of Magento/Checkout/Api/Data/ShippingInformationInterface.php interface.

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$addressInformation = $objectManager->create('Magento\Checkout\Api\Data\ShippingInformationInterface');
$extAttributes = $addressInformation->getExtensionAttributes();

$selectedShipping = $extAttributes->getCustomShippingCharge(); //get custom attribute data.