Need an example on how to get preferred language from Accept-Language request header

Question

I need a code example or library which would parse Accept-Language header and return me preferred language. RFC2616 states that:

The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request. Language tags are defined in section 3.10.
   Accept-Language = "Accept-Language" ":"
                     1#( language-range [ ";" "q" "=" qvalue ] )
   language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" ) 
Each language-range MAY be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range. The quality value defaults to "q=1".

Further reading shows that there are too many "optional", "should", "may" and other turns of speech that prevent me from reinventing the wheel - all I want to know is what language user prefers, any browser answers this question billion times a day.

Any code snippet in any language (except Lisp and Assembler please) would be helpful.

Many thanks in advance!

Split by comma (and preceding space if available), then interrogate each token. Language is the first part, then (if present) a q-code (preference priority). Finally, sort by q values (my understanding is those without a q are to be understood as "1"). — Brad Christie, Mar 29 '12 at 15:15
@Brad, that's what I'm trying to avoid, but definitely will start doing it if there'll be no ready answers. — andbi, Mar 29 '12 at 15:22
If you're looking for an updated answer for ASP.NET Core, scroll down some :) — Mafii, Feb 22 '22 at 09:21

score 47 · Answer 1 · edited Jun 23 '16 at 11:11

47

Solution:

namespace ConsoleApplication
{
    using System;
    using System.Linq;
    using System.Net.Http.Headers;

    class Program
    {
        static void Main(string[] args)
        {
            string header = "en-ca,en;q=0.8,en-us;q=0.6,de-de;q=0.4,de;q=0.2";
            var languages = header.Split(',')
                .Select(StringWithQualityHeaderValue.Parse)
                .OrderByDescending(s => s.Quality.GetValueOrDefault(1));
        }
    }
}

Result:

enter image description here

edited Jun 23 '16 at 11:11

weston

54,145
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answered Dec 21 '14 at 01:01

Zygimantas

8,547
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4

@weston : what kind of magic this is: .Select(StringWithQualityHeaderValue.Parse) ? – Zygimantas Jun 24 '16 at 07:33
1

Lol, called a method group http://stackoverflow.com/questions/886822/what-is-a-method-group-in-c – weston Jun 24 '16 at 07:54

Manse · Accepted Answer · 2012-03-30T13:29:40.360

Try this, its in PHP but using the same regex i'm sure its adaptable to any language :

$langs = array(); // used to store values

if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
    // break up string into pieces (languages and q factors)
    preg_match_all('/([a-z]{1,8}(-[a-z]{1,8})?)\s*(;\s*q\s*=\s*(1|0\.[0-9]+))?/i', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $lang_parse);

    if (count($lang_parse[1])) {
        // create a list like "en" => 0.8
        $langs = array_combine($lang_parse[1], $lang_parse[4]);

        // set default to 1 for any without q factor
        foreach ($langs as $lang => $val) {
            if ($val === '') $langs[$lang] = 1;
        }

        // sort list based on value 
        arsort($langs, SORT_NUMERIC);
    }
}

produces a sorted array with preferred language first :

Array
(
    [en-ca] => 1
    [en] => 0.8
    [en-us] => 0.6
    [de-de] => 0.4
    [de] => 0.2
)

From example ACCEPT_LANGUAGE header : en-ca,en;q=0.8,en-us;q=0.6,de-de;q=0.4,de;q=0.2

Working example here

Thanks, you're just in time - I was goggling at my stub method always returning "en" )) — andbi, Mar 30 '12 at 13:40

score 1 · Answer 3 · answered Feb 22 '22 at 09:19

Update for ASP.NET Core:

Microsoft has that built in by now. Use the AcceptLanguageHeaderRequestCultureProvider:

IApplicationBuilder builder = ...;
builder.UseRequestLocalization(options => options
  .AddInitialRequestCultureProvider(new AcceptLanguageHeaderRequestCultureProvider()));

you can then get the IRequestCultureFeature through the context:

// probably injected through your DI container
IHttpContextAccessor _accessor = ...;
await _accessor.HttpContext.Features
  .Get<IRequestCultureFeature>().Provider
  .DetermineProviderCultureResult(_accessor.HttpContext);

that returns a ProviderCultureResult?, you can then read the cultures (in form of IList<StringSegment>) from that.

Need an example on how to get preferred language from Accept-Language request header

3 Answers3

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