Wininet not caching compressed content

Question

I have a compressed resource that when viewed in IE, loads from the cache as expected. However when my application loads the same URL, Wininet ignores the cache and always downloads the content from the server. With dynamic content compression disabled on IIS, my application behaves the same as IE (the HTTP GET includes an If-None-Match header.)

What can I do to get my app behave the same as IE?

_httpClient = new HttpClient(new WebRequestHandler
{
    CachePolicy = new HttpRequestCachePolicy(HttpRequestCacheLevel.Default),
    AutomaticDecompression = DecompressionMethods.Deflate | DecompressionMethods.GZip
});

using (Task<HttpResponseMessage> tget = _httpClient.GetAsync(uri, HttpCompletionOption.ResponseHeadersRead, _cancel))
{
    tget.Wait();
    response = tget.Result;
}

The HTTP headers for my application's GET:

GET https://beautykiosktest.coinstar.com/conductor/configuration/files/Promos.xml?kioskid=ENG20130027 HTTP/1.1
Accept: */*
Accept-Language: en-US
User-Agent: ConfigurationService/2.3.0.0
Host: beautykiosktest.coinstar.com
Accept-Encoding: gzip, deflate

HTTP/1.1 200 OK
Cache-Control: max-age=30
Transfer-Encoding: chunked
Content-Type: application/xml; charset=utf-8
Content-Encoding: gzip
ETag: "cHmrX5Fp2Z+ETV/qIXiS2A=="
Vary: Accept-Encoding
Server: Microsoft-IIS/8.5
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Tue, 17 Mar 2015 00:39:18 GMT

IE's GET with a 304 response is what I expect

GET https://beautykiosktest.coinstar.com/conductor/configuration/files/Promos.xml?kioskid=ENG20130027 HTTP/1.1
Accept: image/jpeg, image/gif, image/pjpeg, application/x-ms-application, application/xaml+xml, application/x-ms-xbap, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*
Accept-Language: en-US
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/7.0; SLCC2; .NET CLR 2.0.50727; InfoPath.3; .NET4.0C; .NET4.0E; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET CLR 1.1.4322)
Accept-Encoding: gzip, deflate
Host: beautykiosktest.coinstar.com
If-None-Match: "cHmrX5Fp2Z+ETV/qIXiS2A=="
DNT: 1
Connection: Keep-Alive

HTTP/1.1 304 Not Modified
Cache-Control: max-age=30
ETag: "cHmrX5Fp2Z+ETV/qIXiS2A=="
Server: Microsoft-IIS/8.5
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Tue, 17 Mar 2015 00:42:13 GMT

Answer 1

I ended up filing a ticket with Microsoft to resolve this and found out that there is a bug in .NET 4 which causes this. The problem is that a cache entry populated from a response with a Vary header doesn't get matched properly on the next GET because even though the AutomaticDecompression property is set, .NET hasn't added the Accept-Encoding header into the request before doing a WinINET cache test.

The workaround/fix is to add an "Accept-Encoding" default header to the HttpClient instance before using it.

_httpClient.DefaultRequestHeaders.Add("Accept-Encoding", "gzip, deflate");