How to use post_class function?

by

Use the post_class filter to test multiple conditions:

function wpa_post_class( $classes ) {
    global $post;
    if( ! has_post_thumbnail() ) $classes[] = "no-image";
    if( get_the_content() == "" ) $classes[] = "no-content";
    return $classes;
}
add_filter( 'post_class', 'wpa_post_class' );

Then your html will just be:

<li  id="post-<?php the_ID(); ?>" <?php post_class(); ?>>