Cache whole page exept specific portion (Substitution Control) in Asp.net

The Substitution control lets you create areas on the page that can be updated dynamically and then integrated into a cached page.

OVERVIEW :

Caching a page can dramatically increase the performance of a Web application. However, in some cases you need most of the page to be cached and some fragments within the page to be dynamic. For example, if you create a page of news stories that is entirely static for set periods of time, you can set the entire page to be cached. If you wanted to include a rotating ad banner that changed on every page request, then the part of the page containing the advertisement needs to be dynamic.

To allow you to cache a page but substitute some content dynamically, you can use ASP.NET post-cache substitution. With post-cache substitution, the entire page is output cached with specific parts marked as exempt from caching. In the example of the ad banners, the AdRotator control allows you to take advantage of post-cache substitution so that ads dynamically created for each user and for each page refresh.

There are three ways to implement post-cache substitution:

• Declaratively, using the Substitution control. 
• Programmatically, using the Substitution control API (HttpResponse.WriteSubstitution method). 
• Implicitly, using the AdRotator control.

Substitution Control 

The ASP.NET Substitution control specifies a section of a cached page that is created dynamically rather than cached. You place a Substitution control at the location on the page where you want the dynamic content to appear.

At run time, the Substitution control calls a method that you specify with the MethodName property. The method must return a string, which then replaces the content of the Substitution control. The method must be a static method on the containing Page or UserControl control.

Using the substitution control causes client-side cacheability to be changed to server cacheability, so that the page will not be cached on the client. This ensures that future requests to the page call the method again to generate dynamic content.

You can use the Substitution control to insert dynamic content into the cached page. The Substitution control does not render any markup. Instead, you bind the control to a method on the page or on a parent user control. You create a static method that returns the information that you want to insert into the page.

The method called by the Substitution control must meet the following criteria:

• It must be a static method (shared in Visual Basic).
• It must accept a parameter of type HttpContext.
• It must return a value of type String.

Other controls on the page are not accessible to the Substitution control—that is, you cannot examine or change the value of other controls. However, the code does have access to the current page context by using the parameter passed to it.

When the page runs, the Substitution control calls the method and then substitutes the return value from the method for the Substitution control on the page.

Code Examples of Substitution control

 

The following example shows how to use the Substitution control to create dynamically updated content on a cached page. Code in the page's Load event updates a Label control with the current time. Because the page's cache duration is set to 60 seconds, the text of the Label control does not change even if the page is requested multiple times during the 60-second period. A Substitution control on the page calls the static method GetTime, which returns the current time as a string. Every time that the page is refreshed, the value represented by the Substitution control is updated.

<%@ Page Language="C#" %>
<%@ OutputCache Duration=60 VaryByParam="None" %>





    

    

    

    

WriteSubstitution Method 

In addition to using <asp:substitution> controls to indicate replaceable substitution blocks on a page, you can alternatively use the Response.WriteSubstitution method instead.  This method takes as a parameter a delegate object to a HttpResponseSubstitutionCallback method that you can implement on any class within your application 

(it is not limited to only going against static methods on your code-behind class). 

An advantage of calling the WriteSubstitution method instead of using the Substitution control declaratively is that you can call a method of any arbitrary object, rather than calling a static method of the Page or the UserControl object.

The <asp:substitution> control internally uses this method to wire-up delegates in the code-behind classes of pages.  You can likewise use it within your own controls or pages for maximum control and flexibility.

 AdrotatorControl

The AdRotator server control implements support for post-cache substitution internally. If you place an AdRotator control on your page, it will render unique advertisements on each request, regardless of whether the parent page is cached. As a result, a page that includes an AdRotator control is only cached server-side.