using System;

using System.Globalization;

using System.Linq;

using System.Net.Http.Headers;

using System.Collections.Generic;

​

public class Program

{   

    public static class AcceptLanguageHandler

    {

        private static readonly CultureInfo[] SupportedCultures = {

                CultureInfo.CreateSpecificCulture("en-GB"),

                CultureInfo.CreateSpecificCulture("nl-BE"),

                CultureInfo.CreateSpecificCulture("fr-BE"),

                CultureInfo.CreateSpecificCulture("fr-LU"),

                CultureInfo.CreateSpecificCulture("nl-NL"),

                CultureInfo.CreateSpecificCulture("de-DE"),

                CultureInfo.CreateSpecificCulture("fi-FI"),

                CultureInfo.CreateSpecificCulture("fr-FR"),

                CultureInfo.CreateSpecificCulture("da-DK"),

                CultureInfo.CreateSpecificCulture("nb-NO"),

                CultureInfo.CreateSpecificCulture("sv-SE"),

                CultureInfo.CreateSpecificCulture("es-ES"),

                CultureInfo.CreateSpecificCulture("ga-IE"),

                CultureInfo.CreateSpecificCulture("en-IE"),

                CultureInfo.CreateSpecificCulture("en-INT"),

                CultureInfo.CreateSpecificCulture("pl-PL"),

                CultureInfo.CreateSpecificCulture("hr-HR")

            };

        private static readonly IReadOnlyDictionary<string, CultureInfo> s_cultures = new string[] {

            "en-GB",

            "nl-BE",

            "fr-BE",

            "fr-LU",

            "nl-NL",

            "de-DE",

            "fi-FI",

            "fr-FR",

            "da-DK",

            "nb-NO",

            "sv-SE",

            "es-ES",

            "ga-IE",

            "en-IE",

            "en-INT",

            "pl-PL",

            "hr-HR"

        }.ToDictionary(x => x, x => CultureInfo.CreateSpecificCulture(x));

        private static readonly ILookup<string, CultureInfo> s_twoletters = s_cultures.ToLookup(x => x.Value.TwoLetterISOLanguageName, x => x.Value);

​

​

        public static CultureInfo Get(System.Net.Http.Headers.StringWithQualityHeaderValue[] AcceptLanguage)

        {

            // get culture by looking at the value with the highest q (or use first supported culture as fallback)

            var culture = AcceptLanguage

                .Where(headerValue => SupportedCultures.Any(c => c.Name == headerValue.Value || c.TwoLetterISOLanguageName == headerValue.Value))

                .OrderByDescending(headerValue => headerValue.Quality ?? 1)

                .Select(headerValue => SupportedCultures.Single(c => c.Name == headerValue.Value || c.TwoLetterISOLanguageName == headerValue.Value))

                .FirstOrDefault() ?? SupportedCultures.First();

​

            return culture;

        }

        public static CultureInfo Get2(System.Net.Http.Headers.StringWithQualityHeaderValue[] AcceptLanguage)

        {

            foreach (var acceptedLanguage in AcceptLanguage.OrderByDescending(x => x.Quality ?? 1).Select(x => x.Value)) 

            {

                if (s_cultures.TryGetValue(acceptedLanguage, out var culture)) 

                {

                    return culture;

                }

            }

            foreach (var acceptedLanguage in AcceptLanguage.OrderByDescending(x => x.Quality ?? 1).Select(x => x.Value)) 

            {

                var twoletter = s_twoletters[acceptedLanguage].FirstOrDefault();

                if (twoletter != null)

                {

                    return twoletter;

                }

            }

            return SupportedCultures.First();

        }

    }

    public static void Main()

    {

        var nlBe = new StringWithQualityHeaderValue("nl-BE");

        var enGB = new StringWithQualityHeaderValue("en-GB");

        var enIE = new StringWithQualityHeaderValue("en-IE");

        var enRandom = new StringWithQualityHeaderValue("en-RANDOM");

        var en = new StringWithQualityHeaderValue("en"); 

        var random = new StringWithQualityHeaderValue("random"); 

        Console.WriteLine(AcceptLanguageHandler.Get([nlBe])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get([nlBe, enGB]));  // OK

        Console.WriteLine(AcceptLanguageHandler.Get([enGB])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get([nlBe, enGB, en])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get([random])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get([enIE])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get([enRandom])); // OK

        try

        {

            Console.WriteLine(AcceptLanguageHandler.Get([en])); // Crashes : Unhandled exception. System.InvalidOperationException: Sequence contains more than one matching element

        }

        catch (InvalidOperationException ex)

        {

            Console.WriteLine("Sequence contains more than one matching element");

        }

        Console.WriteLine(AcceptLanguageHandler.Get2([nlBe])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get2([nlBe, enGB]));  // OK

        Console.WriteLine(AcceptLanguageHandler.Get2([enGB])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get2([nlBe, enGB, en])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get2([random])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get2([enIE])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get2([enRandom])); // OK

        Console.WriteLine(AcceptLanguageHandler.Get2([en])); // OK

​

    }

}