Deferred execution in parallel LINQ in .NET C#

If you are familiar with LINQ then you are probably aware of the notion of deferred execution: queries are not carried out until they are needed. This is not different in parallel LINQ either. Let’s see an example:

Set up a data source:

int[] integerArray = new int[100];
for (int i = 0; i < integerArray.Length; i++)
{
	integerArray[i] = i;
}

Define a parallel query:

IEnumerable<double> results =
	integerArray.AsParallel().Select(item =>
	{
		return Math.Sqrt(item);
	});

The query has not been carried out at this point. It is carried out when the following foreach loop starts:

double sum = 0;
foreach (double result in results)
{
	sum += result;
}
Console.WriteLine("Total {0}", sum);

You can force query execution with the same extension methods as in “normal” LINQ, such as ToList, ToArray etc.:

IEnumerable<double> results =
	integerArray.AsParallel().Select(item =>
	{
		return Math.Sqrt(item);
	}).ToList();

In this case the query is executed as soon as it has been defined.

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