Overview

• Parallel LINQ can be used on objects to potentially turn a sequential query into a parallel one.
• Extension methods for using PLINQ are defined in the System.Linq.ParallelEnumerable class.

public List<string> ParallelEncrypt(List<string> data) => data.AsParallel().Select((item) => Encrypt(item)).ToList();

• Using the standard query operator AsParallel(), the runtime begins executing over the items within the data collection and execute an action in parallel.
• The result is a completion time that gets noticeably shorter than the synchronous alternative over a significantly sized set when there are multiple processors.
• Invoking the parallel version simply involves a call to the IEnumerable<T>.AsParallel() extension method.
• Furthermore, the result of calling a parallel standard query operator is a parallel enumerator, usually ParallelQuery<T>, which means that further operations on the result of a PLINQ query will be performed in parallel.
• PLINQ exceptions are accessible via the InnerExceptions property of the AggregateException.
  • Therefore, wrapping a PLINQ query in a try/catch block with the exception type of System.AggregateException will successfully handle any exceptions within each iteration that were unhandled.
• You can limit the amount of parallelism that is used with the WithDegreeOfParallelism method.
  • You pass that method an integer that represents the number of processors that you want to use.
  • Normally, PLINQ uses all processors (up to 64).
• Parallel processing does not guarantee any particular order.
  • The results of the parallel code vary depending on the amount of CPUs that are available.
  • AsOrdered specifies that PLINQ should preserve the ordering of the source sequence for the rest of the query, or until the ordering is changed.
  • The query is still processed in parallel, but the results are buffered and sorted.

var numbers = Enumerable.Range(1, 20)
    .AsParallel()
    .AsOrdered()
    .Where(x => x % 5 == 0);
foreach (var number in numbers)
    Console.WriteLine(number);

• If you have a complex query that can benefit from parallel processing but also has some parts that should be done sequentially, use the AsSequential.
  • It specifies that the rest of the query should be run sequentially, as a non-parallel LINQ query.
  • One scenario where this is required is to preserve the ordering of your query.

var parallelResult = Enumerable.Range(1, 20)
	.AsParallel()
	.Where(x => x % 2 == 0)
	.AsSequential();
foreach (var number in parallelResult.Take(5))
	Console.WriteLine(number);

• Use the ForAll operator to iterate over a collection when the iteration can also be done in a parallel way.
• It is a multithreaded enumeration method that enables results to be processed in parallel without first merging back to the consumer thread.
• In contrast to foreach, ForAll does not need all results before it starts executing.

Enumerable.Range(1, 20)
	.AsParallel()
	.Where(x => x % 2 == 0)
	.ForAll(x => Console.WriteLine(x));

Canceling a PLINQ Query

• Like the parallel loops, canceled PLINQ queries will throw a System.OperationCanceledException.