هنگام فراخواني Cookie ها بايد از Request استفاده كنيد نه Response!
در رويداد ButtonShow_Click كدتون رو به صورت زير اصلاح كنيد.
foreach (string CookieName in Request.Cookies)
{
HttpCookie CookieObj = Request.Cookies[CookieName];
Response.Write("Cookie Name: " + CookieObj.Name + "<br/>");
Response.Write("Cookie Value: " + CookieObj.Value + "<br/>");
Response.Write("Cookie Path: " + CookieObj.Path + "<br/>");
Response.Write("Cookie Secure: " + CookieObj.Secure + "<br/>");
Response.Write("----------<br/>");
}