//第一种写法
Response.Cookies["username"].Value = "jack.feng";
Response.Cookies["username"].Expires = DateTime.Now.AddDays(-1);
//第二种写法
HttpCookie acookie = new HttpCookie("username");
acookie.Value = DateTime.Now.ToString();
acookie.Expires = DateTime.Now.AddDays(-1);
Response.Cookies.Add(acookie);