Parallel LINQ in .NET C#: keeping the order

In this post we saw a simple example of PLINQ. We saw that the items are processed in an arbitrary order.

Consider the following:

int[] sourceData = new int[10];
for (int i = 0; i < sourceData.Length; i++)
{
	sourceData[i] = i;
}

IEnumerable<int> parallelResults =
	from item in sourceData.AsParallel()
	where item % 2 == 0
	select item;

foreach (int item in parallelResults)
{
	Console.WriteLine("Item {0}", item);
}

When I ran the code on my machine I got the following output:

0, 4, 6, 8, 2

What if you want the items to be processed in an ascending order? Just append the AsOrdered() extension method:

IEnumerable<int> parallelResults =
	from item in sourceData.AsParallel().AsOrdered()
	where item % 2 == 0
	select item;

This produces the same order as a sequential query execution but with the benefits of parallel execution. However, there’s a small cost to this. Without ordering PLINQ could freely decide how to start and optimise the threads. Now there’s a performance cost to restore the order with the AsOrdered extension method. Keep this in mind and only use ordering if it really matters. Otherwise you can order the resulting list with “normal” LINQ afterwards.

View the list of posts on the Task Parallel Library here.