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$(document).ready(function(){
  $("#myForm").validate({
    rules: {
               myDropDown:{
                  required: function (element) {
                     if($("#myCheckbox").is(':checked'))
                     {
                         var e = document.getElementById("myDropDown");
                         if(e.options[e.selectedIndex].value=="")
                         {
                            return true;  
                         } 
                         else
                         {
                            return false;
                         } 
                     }
                     else
                     {
                         return false;
                     }  
                  }  
               }
           }
  });
});