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// WP_Query arguments
$args = array (
	'post_type'              => array( 'slider' ),
	'posts_per_page'         => '-1',
);

// The Query
$slider = new WP_Query( $args );

// The Loop
if ( $slider->have_posts() ) {
	while ( $slider->have_posts() ) {
		$slider->the_post();
		// do something
	}
} else {
	// no posts found
}

// Restore original Post Data
wp_reset_postdata();