How to get all IANA Timezones from an offset in NodaTime?

In our ASP.NET MVC 5 application we have user profiles with a Timezone ID (IANA). 

public class User
{
    public int Id { get; set; }
    public string TimeZoneId { get; set; }
}

The task is to send all users an email at 6 AM local time. Our plan is to send out emails every hour to all timezones where the local time is 6 AM at that point in time (calculated based on the UTC time of the server).

Since we only have the Timezone ID to go by, I was hoping to get the corresponding Timezone IDs for, let's say, offset -4:00:00 -- taking into account DST.

string[] timezoneIds;

I could then query my database like so:

db.Users.Where(x => timezoneIds.Contains(x.TimeZoneId));

My question is, obviously, is this a decent idea or are there best practices for this problem that I am not aware of?

Thanks.

Jon Skeet
people
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Simpler code for the "building a dictionary" that Serge shows: use LINQ's lookup, DateTimeZone.GetUtcOffset, and Offset keys instead of TimeSpan values:

var now = SystemClock.Instance.Now;
var provider = DateTimeZoneProviders.Tzdb;
var idsByCurrentOffset = provider.Ids.ToLookup(id => provider[id].GetUtcOffset(now));

Then you can use:

var offset = Offset.FromHours(5);
foreach (var id in idsByCurrentOffset[offset])
{
    ...
}

(You don't even need to check for existence first, as the lookup indexer returns an empty sequence when presented with a key that doesn't exist in the lookup.)

people

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