Using AsSequential in order to preserve order

Question

I am looking at this code

var numbers = Enumerable.Range(0, 20);
var parallelResult = numbers.AsParallel().AsOrdered()
    .Where(i => i % 2 == 0).AsSequential();

foreach (int i in parallelResult.Take(5))
    Console.WriteLine(i);

The AsSequential() is supposed to make the resulting array sorted. Actually it is sorted after its execution, but if I remove the call to AsSequential(), it is still sorted (since AsOrdered()) is called.

What is the difference between the two?

Answer 1

AsSequential is just meant to stop any further parallel execution - hence the name. I'm not sure where you got the idea that it's "supposed to make the resulting array sorted". The documentation is pretty clear:

Converts a ParallelQuery into an IEnumerable to force sequential evaluation of the query.

As you say, AsOrdered ensures ordering (for that particular sequence).

Answer 2

I know that this was asked over a year old but here are my two cents.

In the example exposed, i think it uses AsSequential so that the next query operator (in this case the Take operator) it is execute sequentially.

However the Take operator prevent a query from being parallelized, unless the source elements are in their original indexing position, so that is why even when you remove the AsSequential operator, the result is still sorted.