When are interpolated string evaluated?

Question

I have following code:

    internal class Constants
    {
        internal static string Source { get; set; }

        #region EvaluationRepository
        internal static string QUERY_001 = $@"
select
  e.*
from {Source} e
where
  e.id = @Id
";

        internal static string QUERY_002 = $@"
select
  e.*
from {Source} e
where
  e.statusid=@StatusId
  and e.begindate >= @FromDate
  and e.enddate <= @ToDate
";

        internal static string QUERY_003
        {
            get
            {
                return $@"
select
  d.statusid [StatusId],
  count(1) [Count]
from
  (select e.statusid
  from {Source} e
  where e.begindate >= @FromDate and e.enddate <= @ToDate) d
group by d.statusid
";
            }
        }
        #endregion
    }

The only time {Source} is filled is when I expose the query as a property . (QUERY_003)
It doesn't work when exposed as a field . (QUERY_001, QUERY_002)

Can anyone explain why? Because of the staticness (not sure if that's a word)?

Sorry for the verbatim interpolation noise for the SQL :)

Answer 1

It's done at runtime. It's equivalent (down to the IL generated) to using string.Format .

The reason the fields are not properly filled is because Source is empty when it initially executes (when the static class is initialized).