Custom WordPress widget showing CPT entries from the same category/categories.

plugin.php

// Widgets
include_once( BE_DIR . '/lib/widgets/widget-related-case-studies.php' );

widget-related-case-studies.php

<?php

/**
 * Adds SK_Related_Case_Studies_Widget widget.
 */
class SK_Related_Case_Studies_Widget extends WP_Widget {

	/**
	 * Register widget with WordPress.
	 */
	function __construct() {
		parent::__construct(
			'related_case_studies', // Base ID
			__( 'Related Case Studies', 'text_domain' ), // Name
			array( 'description' => __( 'Displays the latest 2 Case Studies belonging to the same category/categories that current single Post belongs to', 'text_domain' ), ) // Args
		);
	}

	/**
	 * Front-end display of widget.
	 *
	 * @see WP_Widget::widget()
	 *
	 * @param array $args     Widget arguments.
	 * @param array $instance Saved values from database.
	 */
	public function widget( $args, $instance ) {

		if ( ! is_singular( 'post' ) ) {
			return;
		}

		$categories = get_the_category();

		$cat_slugs = '';

		foreach( $categories as $category ) { // concatenate
			$cat_slugs .= ',' . $category->category_nicename;
		}

		// WP_Query arguments
		$args_related_case_studies = array (
			'post_type'              => 'case-study',
			'category_name'          => $cat_slugs,
			'posts_per_page'         => '2',
		);

		// The Query
		$query = new WP_Query( $args_related_case_studies );

		// The Loop
		if ( $query->have_posts() ) {
			echo $args['before_widget'];
			if ( ! empty( $instance['title'] ) ) {
				echo $args['before_title'] . apply_filters( 'widget_title', $instance['title'] ). $args['after_title'];
			}
			echo '<ul>';
			while ( $query->have_posts() ) {
				$query->the_post(); ?>
				<!-- Display the Title as a link to the Post's permalink. -->
				<li>
					<a href="<?php the_permalink(); ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a>
					
					<?php if ( has_post_thumbnail() ) {
						$image = genesis_get_image( 'format=url&size=carousel' );
						printf( '<div class="related-case-study-image"><a href="%s" rel="bookmark"><img src="%s" alt="%s" /></a></div>', get_permalink(), $image, the_title_attribute( 'echo=0' ) );
						}
					?>					
				</li>
	 		<?php }
			echo '</ul>';
			echo $args['after_widget'];
		} else {
			// no posts found
			return;
		}

		// Restore original Post Data
		wp_reset_postdata();
		
	}

	/**
	 * Back-end widget form.
	 *
	 * @see WP_Widget::form()
	 *
	 * @param array $instance Previously saved values from database.
	 */
	public function form( $instance ) {
		$title = ! empty( $instance['title'] ) ? $instance['title'] : __( 'Case Studies', 'text_domain' );
		?>
		<p>
		<label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label> 
		<input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>">
		</p>
		<?php 
	}

	/**
	 * Sanitize widget form values as they are saved.
	 *
	 * @see WP_Widget::update()
	 *
	 * @param array $new_instance Values just sent to be saved.
	 * @param array $old_instance Previously saved values from database.
	 *
	 * @return array Updated safe values to be saved.
	 */
	public function update( $new_instance, $old_instance ) {
		$instance = array();
		$instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';

		return $instance;
	}

} // class SK_Related_Case_Studies_Widget

// register SK_Related_Case_Studies_Widget widget
function sk_register_related_case_studies_widget() {
	register_widget( 'SK_Related_Case_Studies_Widget' );
}
add_action( 'widgets_init', 'sk_register_related_case_studies_widget' );