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HttpResponse.WriteSubstitution(HttpResponseSubstitutionCallback) Method


Allows insertion of response substitution blocks into the response, which allows dynamic generation of specified response regions for output cached responses.

 void WriteSubstitution(System::Web::HttpResponseSubstitutionCallback ^ callback);
public void WriteSubstitution (System.Web.HttpResponseSubstitutionCallback callback);
member this.WriteSubstitution : System.Web.HttpResponseSubstitutionCallback -> unit
Public Sub WriteSubstitution (callback As HttpResponseSubstitutionCallback)



The method, user control, or object to substitute.


The target of the callback parameter is of type Control.


You can use the WriteSubstitution method for post-cache substitution in your output-cached pages. By passing a HttpContext object to a callback method with a prescribed HttpResponseSubstitutionCallback signature, you can replace output cached content at any given location in the page cache. To initiate the replacement, call the WriteSubstitution method, passing it the callback method, which must be thread safe and can be either of the following:

  • A static method on the container page or user control.
  • A static or instance method on another arbitrary object.

On the first request to the page, the WriteSubstitution calls the HttpResponseSubstitutionCallback delegate to produce the output. Then, it adds a substitution buffer to the response, which retains the delegate to call on future requests. Finally, it degrades client-side cacheability from public to server-only, ensuring future requests to the page re-invoke the delegate by not caching on the client.


Post-cache substitution is not supported for a cached user control where output caching is applied at the user control level. This is also known as fragment caching. For more information, see Caching Portions of an ASP.NET Page.

Applies to

See also