How to: Use Parallel.Invoke to Execute Parallel Operations

.NET Framework (current version)

This example shows how to parallelize operations by using Invoke in the Task Parallel Library. Three operations are performed on a shared data source. Because none of the operations modifies the source, they can be executed in parallel in a straightforward manner.

System_CAPS_ICON_note.jpg Note

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

' How to: Use Parallel.Invoke to Execute Parallel Operations
Option Explicit On
Option Strict On

Imports System.Threading.Tasks
Imports System.Net

Module ParallelTasks

    Sub Main()
        ' Retrieve Darwin's "Origin of the Species" from
        Dim words As String() = CreateWordArray("")

        '#Region "ParallelTasks"
        ' Perform three tasks in parallel on the source array
                            Console.WriteLine("Begin first task...")
                            ' close first Action
                        End Sub,
                Console.WriteLine("Begin second task...")
                'close second Action
            End Sub,
                Console.WriteLine("Begin third task...")
                GetCountForWord(words, "species")
                'close third Action
            End Sub)
        'close parallel.invoke
        Console.WriteLine("Returned from Parallel.Invoke")
        '#End Region

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

#Region "HelperMethods"
    Sub GetCountForWord(ByVal words As String(), ByVal term As String)
        Dim findWord = From word In words _
            Where word.ToUpper().Contains(term.ToUpper()) _
            Select word

        Console.WriteLine("Task 3 -- The word ""{0}"" occurs {1} times.", term, findWord.Count())
    End Sub

    Sub GetMostCommonWords(ByVal words As String())
        Dim frequencyOrder = From word In words _
            Where word.Length > 6 _
            Group By word
            Into wordGroup = Group, Count()
            Order By wordGroup.Count() Descending _
            Select wordGroup

        Dim commonWords = From grp In frequencyOrder
                          Select grp
                          Take (10)

        Dim s As String
        s = "Task 2 -- The most common words are:" & vbCrLf
        For Each v In commonWords
            s = s & v(0) & vbCrLf
    End Sub

    Function GetLongestWord(ByVal words As String()) As String
        Dim longestWord = (From w In words _
            Order By w.Length Descending _
            Select w).First()

        Console.WriteLine("Task 1 -- The longest word is {0}", longestWord)
        Return longestWord
    End Function

    ' An http request performed synchronously for simplicity.
    Function CreateWordArray(ByVal uri As String) As String()
        Console.WriteLine("Retrieving from {0}", uri)

        ' Download a web page the easy way.
        Dim s As String = New WebClient().DownloadString(uri)

        ' Separate string into an array of words, removing some common punctuation.
        Return s.Split(New Char() {" "c, ControlChars.Lf, ","c, "."c, ";"c, ":"c, _
        "-"c, "_"c, "/"c}, StringSplitOptions.RemoveEmptyEntries)
    End Function
#End Region

    ' Output (May vary on each execution):
    ' Retrieving from
    ' Response stream received.
    ' Begin first task...
    ' Begin second task...
    ' Task 2 -- The most common words are:
    ' species
    ' selection
    ' varieties
    ' natural
    ' animals
    ' between
    ' different
    ' distinct
    ' several
    ' conditions
    ' Begin third task...
    ' Task 1 -- The longest word is characteristically
    ' Task 3 -- The word "species" occurs 1927 times.
    ' Returned from Parallel.Invoke
    ' Press any key to exit 
End Module

Note that with Invoke, you simply express which actions you want to run concurrently, and the runtime handles all thread scheduling details, including scaling automatically to the number of cores on the host computer.

This example parallelizes the operations, not the data. As an alternate approach, you can parallelize the LINQ queries by using PLINQ and run the queries sequentially. Alternatively, you could parallelize the data by using PLINQ. Another option is to parallelize both the queries and the tasks. Although the resulting overhead might degrade performance on host computers with relatively few processors, it would scale much better on computers with many processors.

  • Copy and paste the entire example into a Microsoft Visual Studio 2010 project and press F5.

Parallel Programming
How to: Cancel a Task and Its Children
Parallel LINQ (PLINQ)