In LINQ, do projections off an IOrderedEnumerable<T> preserve the order?

Question

If I have an IOrderedEnumberable<Car>, I sort it and then do a projecting query...

is the order preserved in the projection?

For example, does this scenario work?

IOrderedEnumberable<Car> allCarsOrderedFastestToSlowest = 
            GetAllCars()
                  .OrderByDescending(car=>car.TopSpeed);

var top3FastestCarManufacturers =
            allCarsOrderedFastestToSlowest
                  .Select(car=>car.Manufacturer)
                  .Distinct()
                  .Take(3);

Does the name of the top3FastestCarManufacturers variable convey the meaning of what has really happened in the code?

Answer 1

The documentation for the Distinct method doesn't say anything about whether the order is preserved or not. This is probably because it depends on the underlying implementation of the source.

You can use grouping to get the desired result, by getting the fastest car from each manufacturer, and then get the three fastest from that:

var topThreeFastestCarManufacturers =
  GetAllCars()
  .GroupBy(c => c.Manufacturer)
  .Select(g => g.OrderByDescending(c => c.TopSpeed).First())
  .OrderByDescending(c => c.TopSpeed)
  .Take(3);