simple condition on wordpress function.php not work

i want add a new field "title" on comment of wordpress, after insert the new input field in a default form of wordpress i added this in my function.php for save the title when new comment are submitted.

this is the code i use for save title:

function add_comment_meta_values($idcommento) {

global $post;
$idcommento= get_comment_ID();
$tipodipost= get_post_type($post->ID);

            if( get_post_type($post->ID) == 'service') {



            if(isset($_POST['title_svz']) ) {
        $title= wp_filter_nohtml_kses($_POST['title_svz']);
        add_comment_meta( $idcommento , 'title_svz', $title, false);
            }}

}

add_action ('comment_post', 'add_comment_meta_values', 1);

this code work only when remove the condition :

      if( get_post_type($post->ID) == 'service') {}

and i don't understand why, i have already tried this condition in comment.php or in a footer with simple function like this

function test_function() {

            if( get_post_type($post->ID) == 'service') { echo 'done'; }


}
add_action( 'wp_footer', 'test_function' );

and it's work, so i don't understand why don't work in my primary code, any idea ?

via Chebli Mohamed