Question

If my collection is ordered by date will Distinct() take the first object in list of adjacent duplicates or is it not certain? I am using IEqualityComparer that does not consider date field but I want to be sure the latest date is always taken.

Answer 1

You should use GroupBy:

from s in whatever
group s by new { s.Field1, s.Field2 } into g
select g.OrderByDescending(o => o.Date).First()

EDIT: You can also use your IEqualityComparer with GroupBy:

whatever.GroupBy(
    s => s,      //Key
    g => g.OrderByDescending(o => o.Date).First()  //Result
    new MyComparer()
);