How we can work with output cache programmatically ?

not only cache policy can be created programatically, you can configure it too. more details here

Using Post Caching Substitution in SharePoint 2007 Web Parts

there are scenarios where you might want to implement output caching on your site/page, but have some controls (Web Parts, User Control) excluded from caching so that they dynamically have their content updated on every request.


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