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.

To create dynamic content for a cached page programmatically, you can call the WriteSubstitution method in your page code, passing it the name of a method as a parameter. The method that handles the creation of the dynamic content takes a single HttpContext parameter and returns a string. The return string is the content that will be substituted at the given location. 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.

Calling the WriteSubstitution method 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.

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.

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