dont use preference use plugin to extend any core module that is in di.xml

<type name="Magento\Catalog\Controller\Product\View">
    <plugin name="product-cont-test-module" type="Sugarcode\Test\Model\Plugin\Product" sortOrder="10"/>
</type>

and in Product.php

public function aroundExecute(\Magento\Catalog\Controller\Product\View $subject, \Closure $proceed)
{
    echo 'I Am in Local Controller Before <br>';
    $returnValue = $proceed(); // it get you old function return value
    //$name='#'.$returnValue->getName().'#';
    //$returnValue->setName($name);
    echo 'I Am in Local Controller  After <br>';
    return $returnValue;// if its object make sure it return same object which you addition data
}

How to Override Core Block, Model and controller in Magento2

dont use preference use plugin to extend any core module that is in di.xml

<type name="Magento\Catalog\Controller\Product\View">
    <plugin name="product-cont-test-module" type="Sugarcode\Test\Model\Plugin\Product" sortOrder="10"/>
</type>

and in Product.php

public function aroundExecute(\Magento\Catalog\Controller\Product\View $subject, \Closure $proceed)
{
    echo 'I Am in Local Controller Before <br>';
    $returnValue = $proceed(); // it get you old function return value
    //$name='#'.$returnValue->getName().'#';
    //$returnValue->setName($name);
    echo 'I Am in Local Controller  After <br>';
    return $returnValue;// if its object make sure it return same object which you addition data
}

How to Override Core Block, Model and controller in Magento2

dont use preference use plugin to extend any core module that is in di.xml

<type name="Magento\Catalog\Controller\Product\View">
    <plugin name="product-cont-test-module" type="Sugarcode\Test\Model\Plugin\Product" sortOrder="10"/>
</type>

and in Product.php

public function aroundExecute(\Magento\Catalog\Controller\Product\View $subject, \Closure $proceed)
{
    echo 'I Am in Local Controller Before <br>';
    $returnValue = $proceed(); // it get you old function return value
    //$name='#'.$returnValue->getName().'#';
    //$returnValue->setName($name);
    echo 'I Am in Local Controller  After <br>';
    return $returnValue;// if its object make sure it return same object which you addition data
}

dont use preference use plugin to extend any core module that is in di.xml

<type name="Magento\Catalog\Controller\Product\View">
    <plugin name="product-cont-test-module" type="Sugarcode\Test\Model\Plugin\Product" sortOrder="10"/>
</type>

and in Product.php

public function aroundExecute(\Magento\Catalog\Controller\Product\View $subject, \Closure $proceed)
{
    echo 'I Am in Local Controller Before <br>';
    $returnValue = $proceed(); // it get you old function return value
    //$name='#'.$returnValue->getName().'#';
    //$returnValue->setName($name);
    echo 'I Am in Local Controller  After <br>';
    return $returnValue;// if its object make sure it return same object which you addition data
}

How to Override Core Block, Model and controller in Magento2