[iOS][globalization] Incorrect region information when using `English (US)` language and non-US region

[matouskozak]

There seems to be something off with how iOS determines the language-region combination in preferredLanguages. If you select Germany region, you can choose

• English (US): this will result in behavior where the region is incorrectly set to US even though iPhone region is still Germany.

2025-03-17 14:16:09.229227+0000 net9_maui[13431:5122872] CultureInfo.CurrentCulture: en-US
2025-03-17 14:16:09.229416+0000 net9_maui[13431:5122872] CultureInfo.CurrentUICulture: en-US
2025-03-17 14:16:09.229760+0000 net9_maui[13431:5122872] CultureInfo.CurrentUICulture.DisplayName: English (United States)
2025-03-17 14:16:09.231533+0000 net9_maui[13431:5122872] RegionInfo.CurrentRegion.CurrencyEnglishName: US Dollar
2025-03-17 14:16:09.231593+0000 net9_maui[13431:5122872] RegionInfo.CurrentRegion.ISOCurrencySymbol: USD
2025-03-17 14:16:09.231654+0000 net9_maui[13431:5122872] RegionInfo.CurrentRegion.CurrencyNativeName: US Dollar
2025-03-17 14:16:09.231694+0000 net9_maui[13431:5122872] RegionInfo.CurrentRegion.CurrencySymbol: $
2025-03-17 14:16:09.231771+0000 net9_maui[13431:5122872] CultureInfo.CurrentCulture.NumberFormat.CurrencySymbol: $

• English (UK): this will result in correct behavior, i.e., German regional info and English as a language.

2025-03-17 14:13:56.740872+0000 net9_maui[10392:5103515] CultureInfo.CurrentCulture: en-DE
2025-03-17 14:13:56.741067+0000 net9_maui[10392:5103515] CultureInfo.CurrentUICulture: en-DE
2025-03-17 14:13:56.741291+0000 net9_maui[10392:5103515] CultureInfo.CurrentUICulture.DisplayName: English (Germany)
2025-03-17 14:13:56.743050+0000 net9_maui[10392:5103515] RegionInfo.CurrentRegion.CurrencyEnglishName: Euro
2025-03-17 14:13:56.743108+0000 net9_maui[10392:5103515] RegionInfo.CurrentRegion.ISOCurrencySymbol: EUR
2025-03-17 14:13:56.743168+0000 net9_maui[10392:5103515] RegionInfo.CurrentRegion.CurrencyNativeName: Euro
2025-03-17 14:13:56.743215+0000 net9_maui[10392:5103515] RegionInfo.CurrentRegion.CurrencySymbol: €
2025-03-17 14:13:56.743277+0000 net9_maui[10392:5103515] CultureInfo.CurrentCulture.NumberFormat.CurrencySymbol: €

We should always return regional information for Germany as the iPhone region is set that way.

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @ivanpovazan, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-globalization
See info in area-owners.md if you want to be subscribed.

[makazeu]

When the language is set to English (UK), why does CultureInfo.CurrentCulture show as en-DE? Shouldn't it be en-GB instead?

[matouskozak]

When the language is set to English (UK), why does CultureInfo.CurrentCulture show as en-DE? Shouldn't it be en-GB instead?

It is still unclear to me why we're seeing this difference behavior between English (UK) and English (US). We're taking the current locale (culture) information directly from NSLocale.preferredLanguages Objective-C API. We might need to revisit if this API is the most appropriate and if something like NSLocale.currentLocale doesn't align better with our use-case.

[makazeu]

Region should be different from preferred language, because they're simply different things.

For Windows, the current region is the UserGeo from Win32 API:

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.Windows.cs

Lines 157 to 183 in 1151925

	internal static unsafe CultureData GetCurrentRegionData()
	{
	Span<char> geoIso2Letters = stackalloc char[10];
	int geoId = Interop.Kernel32.GetUserGeoID(Interop.Kernel32.GEOCLASS_NATION);
	if (geoId != Interop.Kernel32.GEOID_NOT_AVAILABLE)
	{
	int geoIsoIdLength;
	fixed (char* pGeoIsoId = geoIso2Letters)
	{
	geoIsoIdLength = Interop.Kernel32.GetGeoInfo(geoId, Interop.Kernel32.GEO_ISO2, pGeoIsoId, geoIso2Letters.Length, 0);
	}
	if (geoIsoIdLength != 0)
	{
	geoIsoIdLength -= geoIso2Letters[geoIsoIdLength - 1] == 0 ? 1 : 0; // handle null termination and exclude it.
	CultureData? cd = GetCultureDataForRegion(geoIso2Letters.Slice(0, geoIsoIdLength).ToString(), true);
	if (cd != null)
	{
	return cd;
	}
	}
	}
	// Fallback to current locale data.
	return CultureInfo.CurrentCulture._cultureData;
	}

However, for all Unix-like OS's, the current region is simply CurrentCulture:

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.Unix.cs

Line 53 in a45130e

internal static unsafe CultureData GetCurrentRegionData() => CultureInfo.CurrentCulture._cultureData;

On iOS, the CurrentCulture is the Locale from Native API:

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.iOS.cs

Lines 25 to 30 in a45130e

	private static string GetLocaleInfoNative(string localeName, LocaleStringData type, string? uiLocaleName = null)
	{
	Debug.Assert(localeName != null, "[CultureData.GetLocaleInfoNative] Expected localeName to be not be null");
	return Interop.Globalization.GetLocaleInfoStringNative(localeName, (uint)type, uiLocaleName);
	}

[matouskozak]

On iOS, the CurrentCulture is the Locale from Native API:

I think the CurrentCulture on iOS is taken from

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.Icu.cs

Lines 178 to 185 in 69c8a1d

	#if TARGET_MACCATALYST || TARGET_IOS || TARGET_TVOS
	if (GlobalizationMode.Hybrid)
	{
	windowsName = Interop.Globalization.GetDefaultLocaleNameNative();
	return windowsName != null && windowsName.Length > 0;
	}
	else
	#endif

I agree that it does seem a bit off to use preferredLanguages API to get CurrentCulture region info. @mdh1418 @steveisok do you remember why we switched from using NSLocale.currentLocale to NSLocale.preferredLanguages in #95786? I suppose that NSLocale.preferredLanguages is what usually changes for users on iPhones but I don't now for sure as I would expect NSLocale.currentLocale to work as well (since we use it on macOS also).

[mdh1418]

@matouskozak No, I don't know any other reason besides trying to fix the issue that the PR targeted. Maybe we should split it into separate functions, one using the preferredLanguage and the other to use the currentLocale.

[tipa]

I found a perhaps related problem. Given these macOS settings, I get CultureInfo.CurrentCulture.Name = "en-DE". I think this is the expected behavior, right?

In a similar setup on iOS (see screenshot below), I get CultureInfo.CurrentCulture.Name = "en-DE" when running the app in Debug mode BUT CultureInfo.CurrentCulture.Name = "en" when running the app in Release mode (Native AOT). This causes incorrect results when trying to format (double.ToString()) or parse decimal values (double.TryParse()), among other things.

• Is this problem covered by this issue as well?
• Is it safe to construct my own CultureInfo with new CultureInfo(CultureInfo.CurrentCulture.TwoLetterISOLanguageName + "-" + NSLocale.CurrentLocale.RegionCode) until this issue is (hopefully) fixed with .net10 or could this cause problems in certain situations?

[matouskozak]

In a similar setup on iOS (see screenshot below), I get CultureInfo.CurrentCulture.Name = "en-DE" when running the app in Debug mode BUT CultureInfo.CurrentCulture.Name = "en" when running the app in Release mode (Native AOT).

This is a new and different issue I haven't seen before (different behavior in debug and release builds). Could you please open an issue for it with a repro steps?

About the workaround, yes, you could also NSLocale.CurrentLocale.LanguageCode to get the language from the device if that fits your purpose.