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HttpCacheVaryByContentEncodings Class

Provides a type-safe way to set the VaryByContentEncodings property of the HttpCachePolicy class.

Namespace:  System.Web
Assembly:  System.Web (in System.Web.dll)

[AspNetHostingPermissionAttribute(SecurityAction.LinkDemand, Level = AspNetHostingPermissionLevel.Minimal)]
public sealed class HttpCacheVaryByContentEncodings

The VaryByContentEncodings property is used to specify whether dynamically compressed responses are cached. Caching dynamically compressed responses means that the cost of compression is incurred only one time, during the first request for the resource (or after an application restart) and when the cache item expires.

The VaryByContentEncodings property of the HttpCachePolicy class identifies which request header parameters ASP.NET uses to uniquely identify a variation of the response if there are multiple cached responses for a resource. This is useful when a response depends on a set of client inputs, such as headers, parameters, or content encodings.

The HttpCacheVaryByContentEncodings class is not directly related to HTTP cache-control headers. However, it helps make sure that a response to a client or a proxy varies by the specified content encoding. Setting the cache to vary by content encoding enables ASP.NET to vary the response by the Accept-Encoding header. When a request is processed, the Accept-Encoding header is checked and the first acceptable encoding is identified and used to take one of the following actions:

  • If a matching encoding is also found in the VaryByContentEncodings list and a cached response exists, the cached response is sent.

  • If a matching encoding is also found in the VaryByContentEncodings list but a cached response does not exist, a response is generated and inserted into the cache.

  • If a matching encoding is not found in the VaryByContentEncodings list, the cache is searched for a non-encoded response, also referred to as the identity response. If the identity response is found, it is sent. Otherwise a new non-encoded response is generated and stored in the cache.

For more information about the VaryByContentEncodings property, see RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1, available on the World Wide Web Consortium (W3C) Web site. See section 14, "Header Field Definitions", for complete details.

You can set the VaryByContentEncodings property by using the @ OutputCache directive or by adding a profile to the outputCacheProfile element in the Web.config file.

The following example shows how to enable a dynamically compressed response that can be served from the output cache. The encoding that is acceptable is "gzip" and is set by using the VaryByContentEncodings attribute of the @ OutputCache directive. If the Web server that hosts the page does not have dynamic compression enabled, the output cache will not have a cached response for the specified content encoding.

<%@ Page Language="C#" %>
<%@ OutputCache VaryByParam="none" Duration="10" VaryByContentEncodings="gzip" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"  

<script runat="server">


<html xmlns="">
<head runat="server">
    <title>Varying Output Cache By Content Encoding</title>
    <form id="form1" runat="server">
    <%= DateTime.Now.ToString() %>


Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5 SP1, 3.0 SP1, 2.0 SP1