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The HttpCachePolicy.VaryByContentEncodings property returns a list of Content-Encoding headers that represent compression schemes to vary the output cache by. Caching dynamically compressed responses means that the cost of compression is incurred only one time, during the first request for the resource.
The HttpCachePolicy.VaryByContentEncodings property is used with the Accept-Encoding header of a request to determine how to serve cached responses for different content encodings that are dynamically compressed. The Accept-Encoding header lists the encodings that the client can decompress.
When you set the cache to vary by content encoding, it lets the response vary indirectly by the value in 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 found in the HttpCachePolicy.VaryByContentEncodings list and a cached response exists, the cached response is sent.
If a matching encoding is found in the HttpCachePolicy.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 HttpCachePolicy.VaryByContentEncodings list, the cache is searched for a non-encoded response, also referred to as the identity response. If the non-encoded response is found, it is sent. Otherwise, a new non-encoded response is generated, sent, and stored in the cache.
For more information about the HttpCachePolicy.VaryByContentEncodings property, see "RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1," which is available on the tp://go.microsoft.com/fwlink/?linkid=37125. Information about content encodings is in section 14, "Header Field Definitions."
You can set the HttpCachePolicy.VaryByContentEncodings property by using the VaryByContentEncodings attribute of the @ OutputCache directive. You can also add a cache profile to the outputCacheProfile element in the Web.config file.