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How to: Create and Execute a Simple PLINQ Query

The following example shows how to create a simple Parallel LINQ query by using the AsParallel extension method on the source sequence, and executing the query by using the ForAll(Of TSource) method.

Note Note

This documentation uses lambda expressions to define delegates in PLINQ. If you are not familiar with lambda expressions in C# or Visual Basic, see Lambda Expressions in PLINQ and TPL.

Imports System.Linq

Module Example
   Public Sub Main()
      Dim source = Enumerable.Range(100, 20000)

      ' Result sequence might be out of order. 
      Dim parallelQuery = From num In source.AsParallel()
                          Where num Mod 10 = 0
                          Select num

      ' Process result sequence in parallel
      parallelQuery.ForAll(Sub(e)
                              DoSomething(e)
                           End Sub)

      ' Or use For Each to merge results first 
      ' as in this example, Where results must 
      ' be serialized sequentially through static Console method. 
      For Each n In parallelQuery
         Console.Write("{0} ", n)
      Next 

      ' You can also use ToArray, ToList, etc, as with LINQ to Objects. 
      Dim parallelQuery2 = (From num In source.AsParallel()
                            Where num Mod 10 = 0
                            Select num).ToArray()

      ' Method syntax is also supported 
      Dim parallelQuery3 = source.AsParallel().Where(Function(n)
                                         Return (n Mod 10) = 0
                                         End Function).Select(Function(n)
                                         Return n
                           End Function)

      For Each i As Integer In parallelQuery3
         Console.Write("{0} ", i)
      Next


      Console.WriteLine()
      Console.WriteLine("Press any key to exit...")
      Console.ReadLine()
   End Sub 

   ' A toy function to demonstrate syntax. Typically you need a more 
   ' computationally expensive method to see speedup over sequential queries. 
   Sub DoSomething(ByVal i As Integer)
      Console.Write("{0:###.## }", Math.Sqrt(i))
   End Sub 
End Module

This example demonstrates the basic pattern for creating and executing any Parallel LINQ query when the ordering of the result sequence is not important; unordered queries are generally faster than ordered queries. The query partitions the source into tasks that are executed asynchronously on multiple threads. The order in which each task completes depends not only on the amount of work involved to process the elements in the partition, but also on external factors such as how the operating system schedules each thread. This example is intended to demonstrate usage, and might not run faster than the equivalent sequential LINQ to Objects query. For more information about speedup, see Understanding Speedup in PLINQ. For more information about how to preserve the ordering of elements in a query, see How to: Control Ordering in a PLINQ Query.

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