Cannot use NodaTime's Instant.InZone with the DateTimeZone in a variable with Npgsql's Entity Framework Core provider

Question

So I'm running into a rather odd issue here. I've got a PostgreSQL 14 server and I'm using the Npgsql Entity Framework Core provider to access it. If I have an Entity Framework Core entity that contains a date as a NodaTime Instant type (which becomes timestamp with time zone in the database) and try to project that via .InZone(timeZone).LocalDateTime , I run into 3 situations.

Actual example query would be something like this:

await context.Entities.Select(e => e.Date.InZone(<timeZone>).LocalDateTime).ToListAsync();

(Although it seems that not including the .LocalDateTime portion has no bearing on the below situations, with the exception of when .InZone(<timeZone>) is replaced with .InUtc() .)

The first situation, if in place of <timeZone> I make an explicit call to DateTimeZoneProviders.Tzdb.GetSystemDefault() , then the above query succeeds without issue. (Although oddly, when I look at EFCore's generated SQL, I do not see AT TIME ZONE like the Npgsql docs mention, but I do see it if I was to replace the .InZone(<timeZone>) call with .InUtc() .)

The second situation, if in place of <timeZone> I put in a reference to a static variable containing the result of DateTimeZoneProviders.Tzdb.GetSystemDefault() , then I get an InvalidOperationException with the following message:

The client projection contains a reference to a constant expression of 'NodaTime.TimeZones.CachedDateTimeZone' which is being passed as an argument to the method 'InZone'. This could potentially cause a memory leak; consider assigning this constant to a local variable and using the variable in the query instead. See https://go.microsoft.com/fwlink/?linkid=2103067 for more information.

Given the above, I run into the third situation, if in place of <timeZone> I use a local variable that contains either the result of DateTimeZoneProviders.Tzdb.GetSystemDefault() or the static variable I mentioned above, then I get an InvalidOperationException with the following message:

Expression '@__timeZone_0' in the SQL tree does not have a type mapping assigned.

Am I doing something wrong here? My expected goal is to be able to take the Instant that I'd get from the database's timestamp with time zone and have it be either a ZonedDateTime or LocalDateTime on the client end, without needing to go through extra steps on the client end after obtaining the data from the database. Right now I would have to get the date from the database using .InUtc().LocalDateTime and then after the list has been materialized, I'd be able to do conversions on the returned values.

Answer 1

The NodaTime plugin doesn't support passing parameterized time zones from .NET; you can only use a constant expression such as DateTimeZoneProviders.Tzdb["Europe/Berlin"] ( see these docs for supported translations). You can use this to convert a PG timestamp with time zone into a timestamp without time zone in PG, and then retrieve the result as a timestamp without time zone , which can be read as a NodaTime LocalDateTime.

However, if what you're looking for is to get a ZonedDateTime/LocalDateTime with the time zone of the client machine (where .NET is executing), then doing .InUtc().LocalDateTime is the way to do that - any reason you're trying to avoid it?

A bit more context (as well as a full code sample) could shed some more light on what you're trying to achieve.