The majority [if not all] of the pages in a dynamic website are dynamic. That is, pages that are created on user request. As we all know, dynamic web pages help to provide dynamic content, customized for the user requesting the page [e.g.: the user's home page]. Dynamic pages also help provide dynamic content fetched from a changing data store without the need for the administrator to change the page content every time something changes in the data store [e.g.: Listing of books in a publisher's website]. The disadvantage is the overhead in creating the pages for every user request.

To overcome this, some websites have page creation engines which create all pages in one go and save them as HTML pages which are then served to the users. But this will only help in scenarios where the page content is the same for all requests [user-independent] as in the second example above. The listing of books is the same irrespective of the user requesting the page. Even if there is provision for listing books category wise by providing different category ID values through the querystring, the page output for a particular category of books is the same for all users.

ASP.NET provides support for "caching" which will help us solve this problem to a great extend. It can cache [store in memory] the output generated by a page and will serve this cached content for future requests. And this is useful only in the second scenario described earlier, where the page content is the same for all requests [user-independent]. The caching feature is customizable in various ways and we will see how we can do that as we go through this article.

Caching a page
In order to cache a page's output, we need to specify an @OutputCache directive at the top of the page. The syntax is as shown below:

<%@ OutputCache Duration=5 VaryByParam="None" %>
As you can see, there are two attributes to this directive. They are:

Duration - The time in seconds of how long the output should be cached. After the specified duration has elapsed, the cached output will be removed and page content generated for the next request. That output will again be cached for 10 seconds and the process repeats.
VaryByParam - This attribute is compulsory and specifies the querystring parameters to vary the cache.
In the above snippet, we have specified the VaryByParam attribute as None which means the page content to be served is the same regardless of the parameters passed through the querystring [see Example 1 in the sample download].

If there are two requests to the same page with varying querystring parameters, e.g.: .../PageCachingByParam.aspx?id=12 and .../PageCachingByParam.aspx?id=15] and separate page content is generated for each of them, the directive should be:

<%@ OutputCache Duration=5 VaryByParam="id" %>
The page content for the two requests will each be cached for the time specified by the Duration attribute [see Example 2 in the sample download].

To specify multiple parameters, use semicolon to separate the parameter names. If we specify the VaryByParam attribute as *, the cached content is varied for all parameters passed through the querystring.

Some pages generate different content for different browsers. In such cases, there is provision to vary the cached output for different browsers. The @OutputCache directive has to be modified to:

<%@ OutputCache Duration=5 VaryByParam="id" VaryByCustom="browser" %>
This will vary the cached output not only for the browser but also its major version. I.e., IE5, IE 6, Netscape 4, Netscape 6 will all get different cached versions of the output.

Caching page fragments
Sometimes we might want to cache just portions of a page. For example, we might have a header for our page which will have the same content for all users. There might be