It depends. Are you using the DistinctBy method added in .NET 6, or a different implementation? And does the ORM you're using translated DistinctBy to SQL, or does it evaluate it on the client? If it evaluates it on the client, and you're using the .NET 6 method or something equivalent, then technically it will return the first item it encounters within each group. However, this is an implementation detail, and you cannot rely on it. And since you don't order by the ID, you can't guarantee that the database will return the records in any ID-related order. If you always want the lowest ID, then you need to be explicit:
var jobSequenceSheets = dc.JobSequenceSheets
.Where(jss => jss.JobId == jobId)
.Where(jss => jss.RevisionNumber == maxRev)
.GroupBy(jss => new { jss.Plan, jss.Elevation }, (_, items) => items.OrderBy(jss => jss.ID).First())
.OrderBy(jss => jss.Plan).ThenBy(jss => jss.Elevation)
.ToList();
NB: Depending on your ORM, you might need to stick an .AsEnumerable() between the second .Where(...) and the .GroupBy(...) to force client evaluation of the grouping.
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer
