Post substitution cache wllows us to write dynamica content into web page if the page is served from cache.
Introduction
Post substitution cache allows us to write dynamic content into web page if the page is served from cache and we needed some part of the page should be refresh each time the page is requested by the user. In this case we can use the asp.net Substitution control or use response.WriteSubstitution method to render the certain part each time. In order to achieve this, we need to pass the static method to as a parameter to the HttpResponseSubstitutionCallback callback delegate of Response.WriteSubstitution
object. The asp.net engine will not create any instance of the page if the page is serving through the asp.net cache rather it will call this callback static method if the response is serving though cache..
ASPX page content
<%@ Page Language="C#" AutoEventWireup="true"CodeBehind="CacheTestPage.aspx.cs"Inherits="WebApplication70515.CacheTestPage" %>
<%@ OutputCache Duration="10" VaryByParam="None" %>
<!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></title>
</head>
<body>
<form id="form1" runat="server">
<div>
</div>
<br />
<asp:Substitution ID="Substitution1" MethodName="GetDateString" runat="server" />
<br />
<asp:Label ID="Label1" runat="server" Text="Get DateTime through Server cache"></asp:Label>:-
<asp:Label ID="Label2" runat="server" Text="Label"></asp:Label>
<br />
<asp:Button ID="btnrefresh" runat="server" Text="Refresh" />
</form>
</body>
</html>
In the above code, we have added Substitution control and set static method name as "GetDateString" so that the asp.net will call this static method without creating instance of the page class. These static methods simily returns the date time string
ASPX.cs page's content
public partial class CacheTestPage : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
Label2.Text = DateTime.Now.ToString();
Response.WriteSubstitution(new HttpResponseSubstitutionCallback(GetDateTimeString));
}
public static string GetDateString(HttpContext context)
{
return "Date time using Substitution cache control <br>" + DateTime.Now.ToString();
}
public static string GetDateTimeString(HttpContext context)
{
return "DateTime through response object's write substitute cache method<br>" + DateTime.Now.ToString();
}
}
OutPut of the above code shown below screen shot
If we click second time on refresh button then the datetime on last will not refresh but the first two dates will be refresh using Post-Cache Substitution functionality.
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Conclusion
The above article show how we can use the Post-Cache Substitution functionlity in asp.net.