How to: Use Parallel.Invoke to Execute Parallel Operations
This example shows how to parallelize operations by using ParallelInvoke() 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.
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.
Example
' 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 Gutenberg.org.
Dim words As String() = CreateWordArray("http://www.gutenberg.org/files/2009/2009.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, "species")
'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 ""{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
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 {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 http://www.gutenberg.org/dirs/etext99/otoos610.txt
' 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
namespace ParallelTasks
{
using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Net;
class ParallelInvoke
{
static void Main()
{
// Retrieve Darwin's "Origin of the Species" from Gutenberg.org.
string[] words = CreateWordArray(@"http://www.gutenberg.org/files/2009/2009.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, "species");
} //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 ""{0}"" occurs {1} times.",
term, findWord.Count());
}
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 {0}", longestWord);
return longestWord;
}
// An http request performed synchronously for simplicity.
static string[] CreateWordArray(string uri)
{
Console.WriteLine("Retrieving from {0}", 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
}
/* Output (May vary on each execution):
Retrieving from http://www.gutenberg.org/dirs/etext99/otoos610.txt
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
*/
}
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.
Compiling the Code
- Copy and paste the entire example into a Microsoft Visual Studio 2010 project and press F5.
See Also
Tasks
How to: Wait on One or More Tasks to Complete
How to: Chain Multiple Tasks with Continuations
How to: Cancel a Task and Its Children