!IsPostBack和Cookies用法

public partial class glassku_Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)//如果是页面首次加载，则执行Add语句；再点击Button1_Click事件，执行二次加载，就不再执行Add语句了。
{
DropDownList1.Items.Add("北京");
DropDownList1.Items.Add("上海");
DropDownList1.Items.Add("杭州");
DropDownList1.Items.Add("成都");
DropDownList1.Items.Add("重庆");
//建立单值Cookies
//法一：
HttpCookie mycookie1 = new HttpCookie("c1");
mycookie1.Value = "zhang";
mycookie1.Expires = DateTime.Now.AddDays(1);
Response.Cookies.Add(mycookie1);

//法二：
HttpCookie mycookie2 = new HttpCookie("c2", "zhangyj");
mycookie2.Expires = DateTime.Now.AddDays(1);
Response.Cookies.Add(mycookie2);

// 法三：
Response.Cookies["username"].Value = Server.UrlEncode("张亚京");
Response.Cookies["username"].Expires = DateTime.Now.AddDays(1);

//建立多个Cookies
//法一：
HttpCookie mycookie3 = new HttpCookie("a");
mycookie3.Values["a1"] = "zhang";
mycookie3.Values["a2"] = "yj";
mycookie3.Expires = DateTime.Now.AddDays(1);
Response.Cookies.Add(mycookie3);

// 法二：
Response.Cookies["b"]["b1"] = "zhang";
Response.Cookies["b"]["b2"] = "yajing";
Response.Cookies["b"].Expires = DateTime.Now.AddDays(1);

}
}
protected void Button1_Click(object sender, EventArgs e)
{
Label1.Text = Request.Cookies["c1"].Value + "<br>"
+ Request.Cookies["c2"].Value + "<br>"
+ Server.UrlDecode(Request.Cookies["username"].Value) + "<br>"
+ Request.Cookies["a"].Value + "<br>"
+ Request.Cookies["b"].Value + "<br>"
+ "选择的城市是：" + DropDownList1.SelectedValue.ToString();
}

}