the_content and is_main_query

I am filtering the content with the the_content filter. Everything works perfect, excerpt that my changes are applied to custom queries as well. My changes appear in the sidebar as well if the widget uses a custom query

To counter that, I’m using is_main_query() to target the main query only, but it is not working. Changes are simply still applied to all queries through out. What is funny though, all other conditional checks like is_single() and is_category() is working if I target specific pages, except that all changes affect any other custom query on that page, whether I use is_main_query() or not

Am I missing something here. How do I apply my changes to the main query only using the the_content filter

add_filter('the_content', 'custom_content');

function custom_content($content){

    if(is_main_query()){ // << THIS IS NOT WORKING
        // My custom content that I add to the_content()    
    }
    return $content;
}

To be honest, the function in_the_loop() is what you are looking for:

add_filter( 'the_content', 'custom_content' );

function custom_content( $content ) {
    if ( in_the_loop() ) {
        // My custom content that I add to the_content()    
    }
    return $content;
}

What in_the_loop does is to check if global for $wp_query (that is the main query object) of the current post is -1 < $current_post < $post_count.

That happens when the main query is looping, because before loop starts, current post is -1, and after loop ends, current post is reset to -1 again.

So, if in_the_loop() is true, it means that the main query object is looping, which is what you need in this case (and is the same result of adding the action on loop_start and removing on loop_end, like the answer @ialocin wrote; in fact it works for the same reason, and got my +1).

Benefit of @ialocin’s approach is when you want to target a different query object than main one, because in_the_loop() only works for main query.