asp.net - Unable to Clear Parent Domain Cookie -

i have issue need clear cookie, created parent domain. create cookie in sub1.mydomain.com .mydomain.com set cookie domain. problem want cookie cleared when user signs out. if in sub2.mydomain.com , try clear cookie using below code, sets cookie domain sub2.mydomain.com.

    if (cookies["cookiename"] != null)     {         httpcookie mycookie = new httpcookie("cookiename");         mycookie.domain = "." + hostname;         mycookie.expires = datetime.now.adddays(-1d);         response.cookies.add(mycookie);     } 

i tried setting expires using cookies["cookiename"].expires, did nothing. thanks.