How to ensure “the_content” filter runs only for the main displayed content?

I have a plugin which adds the method to “the_content” filter.

add_filter('the_content', 'myFilteringFunction', 10000);

Within that function I want to add some links on the beginning and the end of the content. But I only need to do it for the “main” content of the displayed page so – not in the any of the widgets, not in the footer, header etc.

Moreover I only want it to be included for the custom post type which I defined in the same plugin. So I figured out that kind of check, thinking it would be enough.

if( is_single() && get_query_var('post_type') == 'myCustomPostType' && is_main_query() )

Unfortunately it’s not working as intended – at least not in every case.

On the page the plugin WP Types is installed, it’s not working (the links are added despite the condition). Why?

Try adding a condition to your filtering function that checks your post type against get_post_type.

if ( 'book' == get_post_type() )

If you wish to apply this filter to pages as well, try is_singular() and include your custom post type(s) as an argument.

is_singular('book');

This will return true if any of the following conditions are true:
is_single()
is_page()
is_attachment()
‘book’ == get_post_type()