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 <?php
/**
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Test\Testmodule\Model;

use \Magento\Quote\Api\CouponManagementInterface;
use \Magento\Framework\Exception\CouldNotDeleteException;
use \Magento\Framework\Exception\CouldNotSaveException;
use \Magento\Framework\Exception\NoSuchEntityException;

/**
* Coupon management object.
*/
class CouponManagement extends \Magento\Quote\Model\CouponManagement
{
/**
 * Quote repository.
 *
 * @var \Magento\Quote\Api\CartRepositoryInterface
 */
protected $quoteRepository;

/**
 * Constructs a coupon read service object.
 *
 * @param \Magento\Quote\Api\CartRepositoryInterface $quoteRepository Quote repository.
 */
public function __construct(
    \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
) {
 
    $this->quoteRepository = $quoteRepository;
 
}

/**
 * {@inheritdoc}
 */
public function get($cartId)
{
    /** @var  \Magento\Quote\Model\Quote $quote */
    $quote = $this->quoteRepository->getActive($cartId);
 
    return $quote->getCouponCode();
}

/**
 * {@inheritdoc}
 */
public function set($cartId, $couponCode)
{ 
    /** @var  \Magento\Quote\Model\Quote $quote */
    $quote = $this->quoteRepository->getActive($cartId);
    if (!$quote->getItemsCount()) {
        throw new NoSuchEntityException(__('Cart %1 doesn\'t contain products', $cartId));
    }
    $quote->getShippingAddress()->setCollectShippingRates(true);

    try {
        $quote->setCouponCode($couponCode);
        $this->quoteRepository->save($quote->collectTotals());
    } catch (\Exception $e) {
        throw new CouldNotSaveException(__('Could not apply coupon code'));
    }
    if ($quote->getCouponCode() != $couponCode) {
        throw new NoSuchEntityException(__('Coupon code is not valid or expired'));
    }
    return true;
}

/**
 * {@inheritdoc}
 */
public function remove($cartId)
{
    /** @var  \Magento\Quote\Model\Quote $quote */
    $quote = $this->quoteRepository->getActive($cartId);
    if (!$quote->getItemsCount()) {
        throw new NoSuchEntityException(__('Cart %1 doesn\'t contain products', $cartId));
    }
    $quote->getShippingAddress()->setCollectShippingRates(true);
    try {
        $quote->setCouponCode('');
        $this->quoteRepository->save($quote->collectTotals());
    } catch (\Exception $e) {
        throw new CouldNotDeleteException(__('Could not delete coupon code'));
    }
    if ($quote->getCouponCode() != '') {
        throw new CouldNotDeleteException(__('Could not delete coupon code'));
    }
    return true;
}
}

    <?php
/**
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Test\Testmodule\Model;

use \Magento\Framework\Exception\LocalizedException;
use \Magento\Quote\Api\CouponManagementInterface;
use \Magento\Framework\Exception\CouldNotDeleteException;
use \Magento\Framework\Exception\CouldNotSaveException;
use \Magento\Framework\Exception\NoSuchEntityException;

/**
 * Coupon management object.
 */
class CouponManagement implements CouponManagementInterface
{
    /**
     * Quote repository.
     *
     * @var \Magento\Quote\Api\CartRepositoryInterface
     */
    protected $quoteRepository;

    /**
     * Constructs a coupon read service object.
     *
     * @param \Magento\Quote\Api\CartRepositoryInterface $quoteRepository Quote repository.
     */
    public function __construct(
        \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
    ) {
        $this->quoteRepository = $quoteRepository;
    }

    /**
     * @inheritDoc
     */
    public function get($cartId)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        return $quote->getCouponCode();
    }

    /**
     * @inheritDoc
     */
    public function set($cartId, $couponCode)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        if (!$quote->getItemsCount()) {
            throw new NoSuchEntityException(__('The "%1" Cart doesn\'t contain products.', $cartId));
        }
        if (!$quote->getStoreId()) {
            throw new NoSuchEntityException(__('Cart isn\'t assigned to correct store'));
        }
        $quote->getShippingAddress()->setCollectShippingRates(true);

        try {
            $quote->setCouponCode($couponCode);
            $this->quoteRepository->save($quote->collectTotals());
        } catch (LocalizedException $e) {
            throw new CouldNotSaveException(__('The coupon code couldn\'t be applied: ' .$e->getMessage()), $e);
        } catch (\Exception $e) {
            throw new CouldNotSaveException(
                __("The coupon code couldn't be applied. Verify the coupon code and try again."),
                $e
            );
        }
        if ($quote->getCouponCode() != $couponCode) {
            throw new NoSuchEntityException(__("The coupon code isn't valid. Verify the code and try again."));
        }
        return true;
    }

    /**
     * @inheritDoc
     */
    public function remove($cartId)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        if (!$quote->getItemsCount()) {
            throw new NoSuchEntityException(__('The "%1" Cart doesn\'t contain products.', $cartId));
        }
        $quote->getShippingAddress()->setCollectShippingRates(true);
        try {
            $quote->setCouponCode('');
            $this->quoteRepository->save($quote->collectTotals());
        } catch (\Exception $e) {
            throw new CouldNotDeleteException(
                __("The coupon code couldn't be deleted. Verify the coupon code and try again.")
            );
        }
        if ($quote->getCouponCode() != '') {
            throw new CouldNotDeleteException(
                __("The coupon code couldn't be deleted. Verify the coupon code and try again.")
            );
        }
        return true;
    }
}

<?php
/**
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Test\Testmodule\Model;
 
use Magento\Framework\Exception\LocalizedException;
use \Magento\Quote\Api\CouponManagementInterface;
use Magento\Framework\Exception\CouldNotDeleteException;
use Magento\Framework\Exception\CouldNotSaveException;
use Magento\Framework\Exception\NoSuchEntityException;

/**
 * Coupon management object.
 */
class CouponManagement implements CouponManagementInterface
{
    /**
     * Quote repository.
     *
     * @var \Magento\Quote\Api\CartRepositoryInterface
     */
    protected $quoteRepository;

    /**
     * Constructs a coupon read service object.
     *
     * @param \Magento\Quote\Api\CartRepositoryInterface $quoteRepository Quote repository.
     */
    public function __construct(
        \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
    ) {
        $this->quoteRepository = $quoteRepository;
    }

    /**
     * @inheritDoc
     */
    public function get($cartId)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        return $quote->getCouponCode();
    }

    /**
     * @inheritDoc
     */
    public function set($cartId, $couponCode)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        if (!$quote->getItemsCount()) {
            throw new NoSuchEntityException(__('The "%1" Cart doesn\'t contain products.', $cartId));
        }
        if (!$quote->getStoreId()) {
            throw new NoSuchEntityException(__('Cart isn\'t assigned to correct store'));
        }
        $quote->getShippingAddress()->setCollectShippingRates(true);

        try {
            $quote->setCouponCode($couponCode);
            $this->quoteRepository->save($quote->collectTotals());
        } catch (LocalizedException $e) {
            throw new CouldNotSaveException(__('The coupon code couldn\'t be applied: ' .$e->getMessage()), $e);
        } catch (\Exception $e) {
            throw new CouldNotSaveException(
                __("The coupon code couldn't be applied. Verify the coupon code and try again."),
                $e
            );
        }
        if ($quote->getCouponCode() != $couponCode) {
            throw new NoSuchEntityException(__("The coupon code isn't valid. Verify the code and try again."));
        }
        return true;
    }

    /**
     * @inheritDoc
     */
    public function remove($cartId)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        if (!$quote->getItemsCount()) {
            throw new NoSuchEntityException(__('The "%1" Cart doesn\'t contain products.', $cartId));
        }
        $quote->getShippingAddress()->setCollectShippingRates(true);
        try {
            $quote->setCouponCode('');
            $this->quoteRepository->save($quote->collectTotals());
        } catch (\Exception $e) {
            throw new CouldNotDeleteException(
                __("The coupon code couldn't be deleted. Verify the coupon code and try again.")
            );
        }
        if ($quote->getCouponCode() != '') {
            throw new CouldNotDeleteException(
                __("The coupon code couldn't be deleted. Verify the coupon code and try again.")
            );
        }
        return true;
    }
}

 <?php
/**
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Test\Testmodule\Model;

use \Magento\Quote\Api\CouponManagementInterface;
use \Magento\Framework\Exception\CouldNotDeleteException;
use \Magento\Framework\Exception\CouldNotSaveException;
use \Magento\Framework\Exception\NoSuchEntityException;

/**
* Coupon management object.
*/
class CouponManagement extends \Magento\Quote\Model\CouponManagement
{
/**
 * Quote repository.
 *
 * @var \Magento\Quote\Api\CartRepositoryInterface
 */
protected $quoteRepository;

/**
 * Constructs a coupon read service object.
 *
 * @param \Magento\Quote\Api\CartRepositoryInterface $quoteRepository Quote repository.
 */
public function __construct(
    \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
) {
 
    $this->quoteRepository = $quoteRepository;
 
}

/**
 * {@inheritdoc}
 */
public function get($cartId)
{
    /** @var  \Magento\Quote\Model\Quote $quote */
    $quote = $this->quoteRepository->getActive($cartId);
 
    return $quote->getCouponCode();
}

/**
 * {@inheritdoc}
 */
public function set($cartId, $couponCode)
{  
    /** @var  \Magento\Quote\Model\Quote $quote */
    $quote = $this->quoteRepository->getActive($cartId);
    if (!$quote->getItemsCount()) {
        throw new NoSuchEntityException(__('Cart %1 doesn\'t contain products', $cartId));
    }
    $quote->getShippingAddress()->setCollectShippingRates(true);

    try {
        $quote->setCouponCode($couponCode);
        $this->quoteRepository->save($quote->collectTotals());
    } catch (\Exception $e) {
        throw new CouldNotSaveException(__('Could not apply coupon code'));
    }
    if ($quote->getCouponCode() != $couponCode) {
        throw new NoSuchEntityException(__('Coupon code is not valid or expired'));
    }
    return true;
}

/**
 * {@inheritdoc}
 */
public function remove($cartId)
{
    /** @var  \Magento\Quote\Model\Quote $quote */
    $quote = $this->quoteRepository->getActive($cartId);
    if (!$quote->getItemsCount()) {
        throw new NoSuchEntityException(__('Cart %1 doesn\'t contain products', $cartId));
    }
    $quote->getShippingAddress()->setCollectShippingRates(true);
    try {
        $quote->setCouponCode('');
        $this->quoteRepository->save($quote->collectTotals());
    } catch (\Exception $e) {
        throw new CouldNotDeleteException(__('Could not delete coupon code'));
    }
    if ($quote->getCouponCode() != '') {
        throw new CouldNotDeleteException(__('Could not delete coupon code'));
    }
    return true;
}
}

di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">

 <preference for="\Magento\Quote\Model\CouponManagement" type="Test\TestOverride\Model\CouponManagement" /> 

</config>

type="Test\TestOverride\Model\CouponManagement" write your vendor and module name

CouponManagement.php

Test\TestOverride\Model\CouponManagement.php

Create module.xml in app/code/Test/Testmodule/etc

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
<module name="Test_Testmodule" setup_version="1.0.1">
</module>
</config>

Create registration.php in app/code/Test/Testmodule and add the following code in it:

<?php
 
\Magento\Framework\Component\ComponentRegistrar::register(
\Magento\Framework\Component\ComponentRegistrar::MODULE,
'Test_Testmodule',
__DIR__
);

Create di.xml in app/code/Test/Testmodule/etc

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">

<preference for="\Magento\Quote\Model\CouponManagement" type="Test\Testmodule\Model\CouponManagement" /> 

</config>

Override CouponManagement.php

create CouponManagement.php in app/code/Test/Testmodule/Model and add the following code in

<?php
/**
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Test\Testmodule\Model;

use Magento\Framework\Exception\LocalizedException;
use \Magento\Quote\Api\CouponManagementInterface;
use Magento\Framework\Exception\CouldNotDeleteException;
use Magento\Framework\Exception\CouldNotSaveException;
use Magento\Framework\Exception\NoSuchEntityException;

/**
 * Coupon management object.
 */
class CouponManagement implements CouponManagementInterface
{
    /**
     * Quote repository.
     *
     * @var \Magento\Quote\Api\CartRepositoryInterface
     */
    protected $quoteRepository;

    /**
     * Constructs a coupon read service object.
     *
     * @param \Magento\Quote\Api\CartRepositoryInterface $quoteRepository Quote repository.
     */
    public function __construct(
        \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
    ) {
        $this->quoteRepository = $quoteRepository;
    }

    /**
     * @inheritDoc
     */
    public function get($cartId)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        return $quote->getCouponCode();
    }

    /**
     * @inheritDoc
     */
    public function set($cartId, $couponCode)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        if (!$quote->getItemsCount()) {
            throw new NoSuchEntityException(__('The "%1" Cart doesn\'t contain products.', $cartId));
        }
        if (!$quote->getStoreId()) {
            throw new NoSuchEntityException(__('Cart isn\'t assigned to correct store'));
        }
        $quote->getShippingAddress()->setCollectShippingRates(true);

        try {
            $quote->setCouponCode($couponCode);
            $this->quoteRepository->save($quote->collectTotals());
        } catch (LocalizedException $e) {
            throw new CouldNotSaveException(__('The coupon code couldn\'t be applied: ' .$e->getMessage()), $e);
        } catch (\Exception $e) {
            throw new CouldNotSaveException(
                __("The coupon code couldn't be applied. Verify the coupon code and try again."),
                $e
            );
        }
        if ($quote->getCouponCode() != $couponCode) {
            throw new NoSuchEntityException(__("The coupon code isn't valid. Verify the code and try again."));
        }
        return true;
    }

    /**
     * @inheritDoc
     */
    public function remove($cartId)
    {
        /** @var  \Magento\Quote\Model\Quote $quote */
        $quote = $this->quoteRepository->getActive($cartId);
        if (!$quote->getItemsCount()) {
            throw new NoSuchEntityException(__('The "%1" Cart doesn\'t contain products.', $cartId));
        }
        $quote->getShippingAddress()->setCollectShippingRates(true);
        try {
            $quote->setCouponCode('');
            $this->quoteRepository->save($quote->collectTotals());
        } catch (\Exception $e) {
            throw new CouldNotDeleteException(
                __("The coupon code couldn't be deleted. Verify the coupon code and try again.")
            );
        }
        if ($quote->getCouponCode() != '') {
            throw new CouldNotDeleteException(
                __("The coupon code couldn't be deleted. Verify the coupon code and try again.")
            );
        }
        return true;
    }
}