Time Zone IANA Ids to/From Windows Ids conversion APIs

[tarekgh]

Background and Motivation

It is planned in 6.0 release to start supporting the IANA time zone Ids on Windows and support Windows Ids on non-Windows platforms (e.g. Linux). That will allow users to use any time zone Id across all platforms without worrying what Id work on what platform.
In addition to that, users need to be able to convert any Windows Id to IANA Id and vice versa. The proposal here is to provide the needed APIs to do that.

As we are using ICU library for supporting IANA Ids and the conversion, we'll be limited to provide this functionality only when running with ICU. i.e. this functionality is not going to be supported in the Globalization Invariant mode nor when enabling NLS mode on Windows. Because of this limitation, the proposed APIs is using TryConvert pattern to allow users check for the conversion failures without throwing any exception.

Proposed API

namespace System
{
    public sealed partial class TimeZoneInfo : IEquatable<TimeZoneInfo?>, ISerializable, IDeserializationCallback
    {
        // Tells if the current TimeZoneInfo Id is Iana Id or not.
        public readonly bool HasIanaId { get; }

        // Try to convert from Iana Id to Windows Id.
        public static bool TryConvertIanaIdToWindowsId(string ianaId, out string windowsId);

        // Try to convert from Windows Id to Iana Id.
        public static bool TryConvertWindowsIdToIanaId(string windowsId, out string ianaId);
        public static bool TryConvertWindowsIdToIanaId(string windowsId, string region, out string ianaId);
    }
}

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[danmoseley]

Cc @mattjohnsonpint

[mattjohnsonpint]

This (along with the other) should help address #18644, and reduce the need for my TimeZoneConverter library. I have functions very similar to these in that library.

[mattjohnsonpint]

To add some supporting details for motivation, there are some scenarios that require such conversion functionality.

• Processing a time zone received from the browser (via Intl.DateTimeFormat().resolvedOptions().timeZone).
• Working with Windows time zone IDs stored in a database or file, from a non-Windows machine.
• Migrating code from older .NET, when it is using Windows IDs and you now want your app or library to be portable.
• Sending time zones to SQL Server when you need its AT TIME ZONE functionality - since that only works with Windows time zone IDs - even for SQL Server on Linux or Azure. If your .NET app runs on Linux, you will have IANA identifiers and will need to convert.
• Same as the last item, but interacting with other databases like PostgreSQL or MySQL, which use IANA identifiers. If your .NET app runs on Windows, you will need to convert.
• Calling any REST API that accepts or returns an IANA identifier when you may be working with Windows identifiers in your application.

I'm sure I'm forgetting several other cases. I believe there was something about wanting support for this in Unity - perhaps something around leaderboard cutoff times and/or multiplayer games with users from various time zones. I can't recall the exact details.

[terrajobst]

Video

• Looks good as proposed but please make sure to properly null-annotate them.

namespace System
{
    public partial class TimeZoneInfo
    {
        public bool HasIanaId { get; }
        public static bool TryConvertIanaIdToWindowsId(string ianaId, out string windowsId);
        public static bool TryConvertWindowsIdToIanaId(string windowsId, out string ianaId);
        public static bool TryConvertWindowsIdToIanaId(string windowsId, string region, out string ianaId);
    }
}

[mattjohnsonpint]

In particular, string? region, where null has the same behavior as the overload without the region.