Wordpress. Woocommerce. Action hook BEFORE adding to cart

What i need to do: I want to run some checks on a product before being added to the cart. More exactly: I want to compare the product i am about to add to the cart, with the ones already added, to see if there are some conflicts. An example: Let's say we have a product named "Both shoes", and a product "left shoe". A user adds "left shoe" to the cart. Then he adds "both shoes". I want to print an error instead of adding the "both shoes": Sorry, but you can't add both shoes if you've added left shoe to the cart. If you want to buy "both shoes", please first remove "left shoe".

I've looked at class-wc-cart.php and i found an action hook at line 811, but it's too late! It's after the product has been added

"do_action( 'woocommerce_add_to_cart', $cart_item_key, $product_id, $quantity, $variation_id, $variation, $cart_item_data );"

The add_to_cart method starts at line 705. http://wcdocs.woothemes.com/apidocs/source-class-WC_Cart.html#705

How can my "product conflict manager" function be hooked before line 801, without hacking woocommerce?

Thank you!


A bit late, but I think you are looking for add to cart validation, which is filterable. Here's an over simplified version:

function so_validate_add_cart_item( $passed, $product_id, $quantity, $variation_id = '', $variations= '' ) {

    // do your validation, if not met switch $passed to false
    if ( 1 != 2 ){
        $passed = false;
        wc_add_notice( __( 'You can not do that', 'textdomain' ), 'error' );
    return $passed;

add_filter( 'woocommerce_add_to_cart_validation', 'so_validate_add_cart_item', 10, 5 );

add_filter( 'woocommerce_add_to_cart_validation', 'E_coding_hub_only_one_in_cart', 99, 2 );

function E_coding_hub_only_one_in_cart( $passed, $added_product_id ) {

// empty cart first: new item will replace previous wc_empty_cart();

// display a message if you like

wc_add_notice( "Sorry, but you can't add both shoes if you've added left shoe to the cart", "notice" );

return $passed;


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