Make Dapper call GetFieldValue<T> instead of GetValue

Hello,

I am using the NodaTime Plugin of Npgsql and it implements "NodaTime.Instant" as the Default Type for timestamptz.
In addition it also provides the possibility to get a "NodaTime.ZonedDateTime" by calling GetFieldValue<T>.

https://www.npgsql.org/doc/types/basic.html
https://www.npgsql.org/doc/types/nodatime.html

Currently when we defined a class like that:
public class TestData
{
public ZonedDateTime TheTime { get; set; }
}

and try to load some data into it by calling:

c.Query<TestData>(@"select now() as the_time;").FirstOrDefault()

An exception is raised:

Unable to cast object of type 'NodaTime.Instant' to type 'NodaTime.ZonedDateTime'.

When we use Npgsql directly and call GetFieldValue<T> the ZonedDateTime mapping works perfectly.
Is there any way to get this to work with Dapper beside modifying the Npgsql.NodaTime Plugin?

Thanks,
michael

[mgravell]

The thing is: as a library, it isn't as easy as just changing to GetFieldValue<T>; a lot of the time, the value that comes back isn't what you think it is, and the library needs to do some munging / casting / converting; if we used GetField<T> throughout, then we start getting exceptions (mostly InvalidCastException) instead of just finding that the result isn't what we expected - unless we use GetFieldValue<object>(), which is pointless. So then we'd need to special case things instead, and need to start detecting both nodataime and the nodataime npgsql plugin. And from there, it starts getting more and more complex.

I'm not saying it can't be done; but: it really isn't as simple as just changing the API call.

[roji]

@mgravell just noting that GetFieldValue would eliminate the boxing of value types, which is another bonus.

[mgravell]

@roji yep, I understand that; it is a mixed bag - but the range of different providers means that we need to be far more selective about when to prefer it. I'd actively prefer to use it. Maybe this is something for the V2 list

[mgravell]

This should now be possible via #1910 - you'll need to call AddTypeMap, optionally specifying the DbType, but specifying true for the new parameter that enables this mode