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How does output caching work in ASP.NET?

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How does output caching work in ASP.NET?
posted Sep 15, 2015 by Sathyasree

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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 5 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.

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answer Sep 24, 2015 by Manikandan J
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