Display a Product Category Dropdown and Redirect To Single Category Upon Selection.

How To add category dropdown Before WooCommerce product loop?

WooCommerce product loop is generally on shop page and category page. on both pages WooCommerce provide hooks before the loop and after the loop.

1. "woocommerce_before_shop_loop"
2. "woocommerce_after_shop_loop"

For adding a category dropdpwn or other texonomy dropdown We will use "woocommerce_before_shop_loop".  

For Both Product loop. (shop page and category page)

function CategorySwitcher() {   
  wc_product_dropdown_categories();
  $category_base=get_option('woocommerce_permalinks')['category_base'];
  wc_enqueue_js( "
   ('#product_cat').change(function () {
    location.href = '".site_url().'/'.$category_base."/' + $(this).val();
   });
  ");
}
add_action('woocommerce_before_shop_loop','CategorySwitcher',100);

For Only Category Archive Page.

function CategorySwitcher() {   
  if ( is_product_category() ) { 
    wc_product_dropdown_categories();
  }
  $category_base=get_option('woocommerce_permalinks')['category_base'];
  wc_enqueue_js( "
   ('#product_cat').change(function () {
    location.href = '".site_url().'/'.$category_base."/' + $(this).val();
   });
  ");
}
add_action('woocommerce_before_shop_loop','CategorySwitcher',100);

For Only Shop Page.

function CategorySwitcher() {   
  if ( is_shop() ) { 
    wc_product_dropdown_categories();
  }
  $category_base=get_option('woocommerce_permalinks')['category_base'];
  wc_enqueue_js( "
   ('#product_cat').change(function () {
    location.href = '".site_url().'/'.$category_base."/' + $(this).val();
   });
  ");
}
add_action('woocommerce_before_shop_loop','CategorySwitcher',100);