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QValues

Utility classes for correctly parsing Accept-Encodings

Utility classes for RFC 2616 compliant parsing of the Accept-Encoding, Accept-Charset and Accept-Language header.

Background

Dave Transom's blog post on What's wrong with Request.Headers["Accept-Encoding"].Contains("gzip")? goes into detail on why parsing Accept-Encoding with String#contains is a malpractice that has some downsides you should be aware of.

His article also contains some nice utility classes that does what thousand of sites do wrong, which is what I have put up here, along with some bug fixes.

Typical usage

/// load encodings from header
QValueList encodings = new QValueList(Request.Headers["Accept-Encoding"]);

/// get the types we can handle, can be accepted and
/// in the defined client preference
QValue preferred = encodings.FindPreferred("gzip", "deflate", "identity");

/// if none of the preferred values were found, but the
/// client can accept wildcard encodings, we'll default
/// to Gzip.
if (preferred.IsEmpty && encodings.AcceptWildcard && encodings.Find("gzip").IsEmpty)
  preferred = new QValue("gzip");

// handle the preferred encoding
switch (preferred.Name)
{
  case "gzip":
      Response.AppendHeader("Content-Encoding", "gzip");
      Response.Filter = new GZipStream(Response.Filter, CompressionMode.Compress);
    break;
  case "deflate":
      Response.AppendHeader("Content-Encoding", "deflate");
      Response.Filter = new DeflateStream(Response.Filter, CompressionMode.Compress);
    break;
  case "identity":
  default:
    break;
}

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