Runtime: API proposal: Extend CultureInfo.GetCultureInfo and new CultureInfo to optionally accept predefined cultures only

[mklement0]

^{Follow-up from dotnet/runtime#16457}

Rationale and Use Cases

Surprisingly, since Windows _10_ and on Unix-like platforms, CultureInfo.GetCultureInfo(<name>) and also new CultureInfo(<name>) implicitly _create_ (custom) cultures, if <name> does not refer to any of the cultures reported by CultureInfo.GetCultures(CultureTypes.AllCultures) - as long as <name> is _formally_ a valid culture name (which varies by platform).

This makes it cumbersome to ensure that a given culture name refers to a _predefined_ culture - manual validation of the name of the CultureInfo instance returned against the list of predefined cultures reported by CultureInfo.GetCultures(CultureTypes.AllCultures) is necessary.

Arguably, throwing an exception if a non-predefined culture name is used should be the _default_ (whereas _creation_ of a custom culture should be a deliberate act), but changing this is no longer an option.

An example use case is a helper function in PowerShell for convenient testing of culture-sensitive functionality, Use-Culture, to which you pass the name of a predefined culture; e.g., Use-Culture de-DE { 1.2 } outputs 1,2, reflecting the culture-specific decimal mark, ,. The intent is to test with _predefined_ cultures, so a _typo_ such as Use-Culture de_DE { 1.2 } should result in an _error_, as opposed to - unintended - ad-hoc creation of a culture.
The proposed API extension would would allow the function to test the validity of a given culture name directly, without having to search through the array of predefined cultures returned by CultureInfo.GetCultures(CultureTypes.AllCultures).

A pending enhancement to the Get-Culture cmdlet would benefit similarly.

Proposed API

Introduce a Boolean predefinedOnly parameter which ensures that only the names of predefined cultures are accepted and throws a System.Globalization.CultureNotFoundException exception otherwise:

public static System.Globalization.CultureInfo GetCultureInfo (string name, bool predefinedOnly);

// Note: Since a CultureInfo (string, bool) constructor already exists, the parameter can only
//           be *added* to it.
public CultureInfo (string name, bool useUserOverride, bool predefinedOnly);

Note that both signatures are needed, because only the constructor form enables retrieval of culture information that reflects user overrides (user-specific tweaks to a predefined culture via Control Panel).

@tarekgh suggests holding off on implementing the following signature until the need arises, if ever:

public static System.Globalization.CultureInfo GetCultureInfo (string name, string altName, bool predefinedOnly);

[tarekgh]

Thanks, @mklement0

Do we really need
```C#
public static System.Globalization.CultureInfo GetCultureInfo (string name, string altName, bool predefinedOnly);

Also, I don't think we can have 
```C#
public CultureInfo (string name, bool predefinedOnly);

because we already have the constructor that takes a string and boolean parameters. this need to change.

Would it be better to have TryGetCultureInfo family which can return bool and not throwing?

```C#

static bool TryeGetCultureInfo(string name, out CultureInfo culture);
static bool TryeGetCultureInfo(string name, bool userOverride, out CultureInfo culture);
static bool TryeGetCultureInfo(string name, bool userOverride, bool predefinedOnly, out CultureInfo culture);

```

[mklement0]

Good point re public CultureInfo (string name, bool predefinedOnly), thanks - I've removed it from the OP.

---

Re whether we need GetCultureInfo (string name, string altName, bool predefinedOnly):

I think so, yes, because it's likely that you want to ensure that altName too refers to an _existing_ culture.

---

While _also_ having TryGetCultureInfo methods wouldn't hurt, I wouldn't _only_ provide that, because if the user's intent to is _retrieve a preexisting culture that they assume indeed exists_, it is awkward to frame that in terms of a _try_ - Try*() methods are for situations where you _anticipate_ failure, which is not the case here.

[tarekgh]

I think so, yes, because it's likely that you want to ensure that altName too refers to an existing culture.

I am asking if you are going to use it? we can expose this later as needed. I want to ensure we expose only what is needed in your scenario.

For TryGetCultureInfo, I'll not add here for now if we are going to expose a new constructors or GetCultureInfo override

[mklement0]

I am asking if you are going to use it?

No, in the context of PowerShell I'm not aware of a need for a GetCultureInfo (string name, string altName) overload that enforces use of preexisting culture names only.

Out of curiosity, however: why not make that change too, for consistency? Almost seems easier than leaving users to wonder about this omission.

[tarekgh]

Out of curiosity, however: why not make that change too, for consistency? Almost seems easier than leaving users to wonder about this omission.

Your scenario is really not the main scenario. I am not expecting many others will use this. this is why I don't want to expose something nobody else will use it. if we get a request later to have it, we can add it at any time.

[tarekgh]

@mklement0 could you please update the top description with the final proposal as we discussed it here? please follow the proposal format (look at the issue as an example https://github.com/dotnet/corefx/issues/28944). thanks.

[mklement0]

@tarekgh, please see if my update to the OP is what you had in mind.

[tarekgh]

please see if my update to the OP is what you had in mind.

LGTM. thanks.

[tarekgh]

cc @krwq

[terrajobst]

Video

After some debate we settled on this:

C# namespace System.Globalization { public partial class CultureInfo { public static CultureInfo GetCultureInfo(string name, bool prefinedOnly); } }

This API would throw for invalid names as well as names that weren't system provided when predefinedOnly is true.

[terrajobst]

Correct. I've fixed the comment.

[tarekgh]

Done by the PR https://github.com/dotnet/runtime/pull/1261