Sessions

Sessions can be used to store even complex data for the user just like cookies. Actually, sessions will use cookies to store the data, unless you explicitly tell it not to. Sessions can be used easily in ASP.NET with the Session object. We will re-use the cookie example, and use sessions instead. Keep in mind though, that sessions will expire after a certain amount of minutes, as configured in the web.config file. Markup code:
<%@ 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>Sessions</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>
And here is the CodeBehind:
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(Session["BackgroundColor"] != null)
        {
            ColorSelector.SelectedValue = Session["BackgroundColor"].ToString();
            BodyTag.Style["background-color"] = ColorSelector.SelectedValue;
        }
    }

    protected void ColorSelector_IndexChanged(object sender, EventArgs e)
    {
        BodyTag.Style["background-color"] = ColorSelector.SelectedValue;
        Session["BackgroundColor"] = ColorSelector.SelectedValue;
    }
}
As you can see, the example doesn't need a lot of changes to use sessions instead of cookies. Please notice that session values are tied to an instance of your browser. If you close down the browser, the saved value(s) will usually be "lost".

Also, if the webserver recycles the aspnet_wp.exe process, sessions are lost, since they are saved in memory as well. This can be avoided by saving session states on a separate StateServer or by saving to a SQL server, but that's beyond the scope of this article.
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