Walking through HTML form elements with jquery script, I see that I'm able to get the tag's id with this code:

$(":checkbox,:radio,:text,:selected", this).each(function(){
    var controlName = $(this).attr("id");
    var controlType = $(this).attr("type");

    alert(controlName);

    if(controlType=="checkbox" || controlType=="radio"){
        if ($(this).attr('checked')=="checked"){cadena += "&" + controlName + "=on";}
        else{ cadena += "&" + controlName + "=off";}    
    }
    else{
        cadena += "&" + controlName + "=" + $(this).attr('value');
    }               
});

It works with checkboxes, radioButtons and textInputs, giving me the right ids. But for select tags it always return "undefined".

What I'm doing wrong?

Thanks.

* UPDATE * This is my current form. I know that it is not a "normal" form, but I'm using it in a special application. Thanks to everybody for your patience:

    <form action="#" id="ejemplo" class="ejemplo">
    <fieldset>
        <legend> Formulario de ejemplo </legend>

        <input id="Nombre" type="text" value="Texto1" name="Nombre" />    <label for="Nombre">Nombre</label> <br/>
        <input id="Apellidos" type="text" value="Texto1"  name="Apellidos" /> <label for="Apellidos">Apellidos</label> <br/>

        <label for="campo2">Campo 2</label>
        <select id="coches">
            <option value="volvo">Volvo</option>
            <option value="saab">Saab</option>
            <option value="mercedes">Mercedes</option>
            <option value="audi">Audi</option>
        </select>

        <br/>

        <input type="checkbox" id="option1"> <label for="option1"> Milk   </label>
        <input type="checkbox" id="option2"> <label for="option2"> Butter </label>
        <input type="checkbox" id="option3"> <label for="option3"> Cheese </label>
        <br/>

        <input type="radio" name="group1" id="radio1"> <label for="radio1"> Milk   </label> <br/>
        <input type="radio" name="group1" id="radio2"> <label for="radio2"> Butter </label> <br/>
        <input type="radio" name="group1" id="radio3"> <label for="radio3"> Cheese </label>
        <br/>

        <textarea cols="40" rows="5" id="areaTexto">Inside de text area!</textarea>
        <br/>
        <input id="boton" type="button" value="Boton"><br>
    </fieldset>
</form>

Getting the select "coche" id, it returns "undefined". But getting its value return the correct one.

After run My code it returns that:

&Nombre=Texto1&Apellidos=Texto1&undefined=volvo&option1=off&option2=off&option3=off&radio1=off&radio2=off&radio3=off

Look at undefined=volvo :S