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How to: Use Parallel.Invoke to Execute Parallel Operations

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

Note

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

Example

namespace ParallelTasks
{
    using System;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    using System.Net;

    class ParallelInvoke
    {
        static void Main()
        {
            // Retrieve Goncharov's "Oblomov" from Gutenberg.org.
            string[] words = CreateWordArray(@"http://www.gutenberg.org/files/54700/54700-0.txt");

            #region ParallelTasks
            // Perform three tasks in parallel on the source array
            Parallel.Invoke(
                () =>
                {
                    Console.WriteLine("Begin first task...");
                    GetLongestWord(words);
                },  // close first Action

                () =>
                {
                    Console.WriteLine("Begin second task...");
                    GetMostCommonWords(words);
                }, //close second Action

                () =>
                {
                    Console.WriteLine("Begin third task...");
                    GetCountForWord(words, "sleep");
                } //close third Action
            ); //close parallel.invoke

            Console.WriteLine("Returned from Parallel.Invoke");
            #endregion

            Console.WriteLine("Press any key to exit");
            Console.ReadKey();
        }

        #region HelperMethods
        private static void GetCountForWord(string[] words, string term)
        {
            var findWord = from word in words
                where word.ToUpper().Contains(term.ToUpper())
                select word;

            Console.WriteLine($@"Task 3 -- The word ""{term}"" occurs {findWord.Count()} times.");
        }

        private static void GetMostCommonWords(string[] words)
        {
            var frequencyOrder = from word in words
                where word.Length > 6
                group word by word into g
                orderby g.Count() descending
                select g.Key;

            var commonWords = frequencyOrder.Take(10);

            StringBuilder sb = new StringBuilder();
            sb.AppendLine("Task 2 -- The most common words are:");
            foreach (var v in commonWords)
            {
                sb.AppendLine("  " + v);
            }
            Console.WriteLine(sb.ToString());
        }

        private static string GetLongestWord(string[] words)
        {
            var longestWord = (from w in words
                orderby w.Length descending
                select w).First();

            Console.WriteLine($"Task 1 -- The longest word is {longestWord}.");
            return longestWord;
        }

        // An http request performed synchronously for simplicity.
        static string[] CreateWordArray(string uri)
        {
            Console.WriteLine($"Retrieving from {uri}");

            // Download a web page the easy way.
            string s = new WebClient().DownloadString(uri);

            // Separate string into an array of words, removing some common punctuation.
            return s.Split(
                new char[] { ' ', '\u000A', ',', '.', ';', ':', '-', '_', '/' },
                StringSplitOptions.RemoveEmptyEntries);
        }
        #endregion
    }
}
//        The example displays output like the following:
//              Retrieving from http://www.gutenberg.org/files/54700/54700-0.txt
//              Begin first task...
//              Begin second task...
//              Begin third task...
//              Task 2 -- The most common words are:
//              Oblomov
//              himself
//              Schtoltz
//              Gutenberg
//              Project
//              another
//              thought
//              Oblomov's
//              nothing
//              replied
//
//              Task 1 -- The longest word is incomprehensible.
//              Task 3 -- The word "sleep" occurs 57 times.
//              Returned from Parallel.Invoke
//              Press any key to exit

Imports System.Net
Imports System.Threading.Tasks

Module ParallelTasks
    Sub Main()
        ' Retrieve Goncharov's "Oblomov" from Gutenberg.org.
        Dim words As String() = CreateWordArray("http://www.gutenberg.org/files/54700/54700-0.txt")

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

        Console.WriteLine("Press any key to exit")
        Console.ReadKey()
    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 ""{term}"" occurs {findWord.Count()} times.")
    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
        Next
        Console.WriteLine(s)
    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 {longestWord}.")
        Return longestWord
    End Function


    ' An http request performed synchronously for simplicity.
    Function CreateWordArray(ByVal uri As String) As String()
        Console.WriteLine($"Retrieving from {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
End Module
' The example displays output like the following:
'       Retrieving from http://www.gutenberg.org/files/54700/54700-0.txt
'       Begin first task...
'       Begin second task...
'       Begin third task...
'       Task 2 -- The most common words are:
'       Oblomov
'       himself
'       Schtoltz
'       Gutenberg
'       Project
'       another
'       thought
'       Oblomov's
'       nothing
'       replied
'
'       Task 1 -- The longest word is incomprehensible.
'       Task 3 -- The word "sleep" occurs 57 times.
'       Returned from Parallel.Invoke
'       Press any key to exit

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 scales better on computers with many processors.

Compile the Code

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

See also