Cookies are small pieces of text, stored on the client's
computer to be used only by the website setting the cookies. This allows
webapplications to save information for the user, and then re-use it on each
page if needed. Here is an example where we save a users choice of background
color:
<%@
Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default"
%>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Cookies</title>
</head>
<body runat="server" id="BodyTag">
<form id="form1" runat="server">
<asp:DropDownList
runat="server" id="ColorSelector" autopostback="true" onselectedindexchanged="ColorSelector_IndexChanged">
<asp:ListItem value="White" selected="True">Select color...</asp:ListItem>
<asp:ListItem value="Red">Red</asp:ListItem>
<asp:ListItem value="Green">Green</asp:ListItem>
<asp:ListItem value="Blue">Blue</asp:ListItem>
</asp:DropDownList>
</form>
</body>
</html>
The page simply contains a DropDownList control, which
automatically posts back each time a new item is selected. It has 3 different
colors, besides the default one, which is simply white. Once a new item is
selected, the ColorSelector_IndexChanged method is fired, from our CodeBehind
file:
using System;
using System.Data;
using System.Web;
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender,
EventArgs e)
{
if(Request.Cookies["BackgroundColor"]
!= null)
{
ColorSelector.SelectedValue = Request.Cookies["BackgroundColor"].Value;
BodyTag.Style["background-color"]
= ColorSelector.SelectedValue;
}
}
protected void
ColorSelector_IndexChanged(object sender, EventArgs e)
{
BodyTag.Style["background-color"]
= ColorSelector.SelectedValue;
HttpCookie cookie = new HttpCookie("BackgroundColor");
cookie.Value =
ColorSelector.SelectedValue;
cookie.Expires =
DateTime.Now.AddHours(1);
Response.SetCookie(cookie);
}
}
Okay, two different parts to be explained here. First, the
Page_Load method, which is called on each page request. Here we check for a
cookie to tell us which background color should be used. If we find it, we set
the value of our dropdown list to reflect this, as well as the background color
of the page, simply by accessing the style attribute of the body tag.
Then we have the ColorSelector_IndexChanged method, which is
called each time the user selects a new color. Here we set the background color
of the page, and then we create a cookie, to hold the value for us. We allow it
to expire after one hour, and then we set it by calling the SetCookie method on
the Response object.
Try running the example, and set a color. Now close the browser, and start it up again. You will see that the choice of color is saved, and it will remain saved for an hour. However, nothing prevents you from saving the choice for much longer. Simply add a bigger value to the expiry date, or set an absolute value for it.
Try running the example, and set a color. Now close the browser, and start it up again. You will see that the choice of color is saved, and it will remain saved for an hour. However, nothing prevents you from saving the choice for much longer. Simply add a bigger value to the expiry date, or set an absolute value for it.
