Parallel.ForEach should not be used for IO bound tasks

docs/standard/parallel-programming/how-to-write-a-simple-parallel-foreach-loop.md

@@ -1,7 +1,7 @@
 ---
 title: Write a simple parallel program using Parallel.ForEach
 description: In this article, learn how to enable data parallelism in .NET. Write a Parallel.ForEach loop over any IEnumerable or IEnumerable<T> data source.
-ms.date: 02/14/2019
+ms.date: 02/23/2021
 dev_langs:
   - "csharp"
   - "vb"
@@ -19,7 +19,7 @@ This example shows how to use a <xref:System.Threading.Tasks.Parallel.ForEach%2A
 
 ## Example
 
-This example assumes you have several .jpg files in a *C:\Users\Public\Pictures\Sample Pictures* folder and creates a new sub-folder named *Modified*. When you run the example, it rotates each .jpg image in *Sample Pictures* and saves it to *Modified*. You can modify the two paths as necessary.
+This example demonstrates <xref:System.Threading.Tasks.Parallel.ForEach%2A?displayProperty=nameWithType> for CPU intensive operations. When you run the example, it randomly generates 2 million numbers and tries to filter to prime numbers. The first case iterates over the collection via a `for` loop. The second case iterates over the collection via <xref:System.Threading.Tasks.Parallel.ForEach%2A?displayProperty=nameWithType>. The resulting time taken by each iteration is displayed when the application is finished.
 
 [!code-csharp[TPL_Parallel#03](../../../samples/snippets/csharp/VS_Snippets_Misc/tpl_parallel/cs/simpleforeach.cs#03)]
 [!code-vb[TPL_Parallel#03](../../../samples/snippets/visualbasic/VS_Snippets_Misc/tpl_parallel/vb/simpleforeach.vb#03)]
@@ -43,14 +43,6 @@ In Visual Studio, there are Visual Basic and C# console application templates fo
 
 From the command line, you can use either the .NET Core CLI commands (for example, `dotnet new console` or `dotnet new console -lang vb`), or you can create the file and use the command-line compiler for a .NET Framework application.
 
-For a .NET Core project, you must reference the **System.Drawing.Common** NuGet package. In Visual Studio, use the NuGet Package Manager to install the package. Alternatively, you can add a reference to the package in your \*.csproj or \*.vbproj file:
-
-```xml
-<ItemGroup>
-     <PackageReference Include="System.Drawing.Common" Version="4.5.1" />
-</ItemGroup>
-```
-
 To run a .NET Core console application from the command line, use `dotnet run` from the folder that contains your application.
 
 To run your console application from Visual Studio, press **F5**.

samples/snippets/csharp/VS_Snippets_Misc/tpl_parallel/cs/simpleforeach.cs

@@ -1,40 +1,84 @@
 // <snippet03>
 using System;
-using System.IO;
-using System.Threading;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
 using System.Threading.Tasks;
-using System.Drawing;
 
-public class Example
+namespace ParallelExample
 {
-    public static void Main()
+    class Program
     {
-        // A simple source for demonstration purposes. Modify this path as necessary.
-        string[] files = Directory.GetFiles(@"C:\Users\Public\Pictures\Sample Pictures", "*.jpg");
-        string newDir = @"C:\Users\Public\Pictures\Sample Pictures\Modified";
-        Directory.CreateDirectory(newDir);
-
-        // Method signature: Parallel.ForEach(IEnumerable<TSource> source, Action<TSource> body)
-        Parallel.ForEach(files, (currentFile) =>
-                                {
-                                    // The more computational work you do here, the greater
-                                    // the speedup compared to a sequential foreach loop.
-                                    string filename = Path.GetFileName(currentFile);
-                                    var bitmap = new Bitmap(currentFile);
-
-                                    bitmap.RotateFlip(RotateFlipType.Rotate180FlipNone);
-                                    bitmap.Save(Path.Combine(newDir, filename));
-
-                                    // Peek behind the scenes to see how work is parallelized.
-                                    // But be aware: Thread contention for the Console slows down parallel loops!!!
-
-                                    Console.WriteLine($"Processing {filename} on thread {Thread.CurrentThread.ManagedThreadId}");
-                                    //close lambda expression and method invocation
-                                });
-
-        // Keep the console window open in debug mode.
-        Console.WriteLine("Processing complete. Press any key to exit.");
-        Console.ReadKey();
+        static void Main()
+        {
+            // 2 million
+            var limit = 2_000_000;
+            var numbers = Enumerable.Range(0, limit).ToList();
+
+            var watch = Stopwatch.StartNew();
+            var primeNumbersFromForeach = GetPrimeList(numbers);
+            watch.Stop();
+
+            var watchForParallel = Stopwatch.StartNew();
+            var primeNumbersFromParallelForeach = GetPrimeListWithParallel(numbers);
+            watchForParallel.Stop();
+
+            Console.WriteLine($"Classical foreach loop | Total prime numbers : {primeNumbersFromForeach.Count} | Time Taken : {watch.ElapsedMilliseconds} ms.");
+            Console.WriteLine($"Parallel.ForEach loop  | Total prime numbers : {primeNumbersFromParallelForeach.Count} | Time Taken : {watchForParallel.ElapsedMilliseconds} ms.");
+
+            Console.WriteLine("Press any key to exit.");
+            Console.ReadLine();
+        }
+
+        /// <summary>
+        /// GetPrimeList returns Prime numbers by using sequential ForEach
+        /// </summary>
+        /// <param name="inputs"></param>
+        /// <returns></returns>
+        private static IList<int> GetPrimeList(IList<int> numbers) => numbers.Where(IsPrime).ToList();
+
+        /// <summary>
+        /// GetPrimeListWithParallel returns Prime numbers by using Parallel.ForEach
+        /// </summary>
+        /// <param name="numbers"></param>
+        /// <returns></returns>
+        private static IList<int> GetPrimeListWithParallel(IList<int> numbers)
+        {
+            var primeNumbers = new ConcurrentBag<int>();
+
+            Parallel.ForEach(numbers, number =>
+            {
+                if (IsPrime(number))
+                {
+                    primeNumbers.Add(number);
+                }
+            });
+
+            return primeNumbers.ToList();
+        }
+
+        /// <summary>
+        /// IsPrime returns true if number is Prime, else false.(https://en.wikipedia.org/wiki/Prime_number)
+        /// </summary>
+        /// <param name="number"></param>
+        /// <returns></returns>
+        private static bool IsPrime(int number)
+        {
+            if (number < 2)
+            {
+                return false;
+            }
+
+            for (var divisor = 2; divisor <= Math.Sqrt(number); divisor++)
+            {
+                if (number % divisor == 0)
+                {
+                    return false;
+                }
+            }
+            return true;
+        }
     }
 }
-// </snippet03>
+// </snippet03>

samples/snippets/csharp/VS_Snippets_Misc/tpl_parallel/cs/simpleforeach.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net5.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

samples/snippets/visualbasic/VS_Snippets_Misc/tpl_parallel/vb/simpleforeach.vb

@@ -1,37 +1,60 @@
 '<snippet03>
-Imports System.IO
-Imports System.Threading
-Imports System.Threading.Tasks
-Imports System.Drawing
-
-Module ForEachDemo
-
-    Sub Main()
-        ' A simple source for demonstration purposes. Modify this path as necessary.
-        Dim files As String() = Directory.GetFiles("C:\Users\Public\Pictures\Sample Pictures", "*.jpg")
-        Dim newDir As String = "C:\Users\Public\Pictures\Sample Pictures\Modified"
-        Directory.CreateDirectory(newDir)
-
-        Parallel.ForEach(files, Sub(currentFile)
-                                    ' The more computational work you do here, the greater 
-                                    ' the speedup compared to a sequential foreach loop.
-                                    Dim filename As String = Path.GetFileName(currentFile)
-                                    Dim bitmap As New Bitmap(currentFile)
-
-                                    bitmap.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone)
-                                    bitmap.Save(Path.Combine(newDir, filename))
-
-                                    ' Peek behind the scenes to see how work is parallelized.
-                                    ' But be aware: Thread contention for the Console slows down parallel loops!!!
-
-                                    Console.WriteLine($"Processing {filename} on thread {Thread.CurrentThread.ManagedThreadId}")
-                                    'close lambda expression and method invocation
-                                End Sub)
-
-
-        ' Keep the console window open in debug mode.
-        Console.WriteLine("Processing complete. Press any key to exit.")
-        Console.ReadKey()
-    End Sub
-End Module
-'</snippet03>
+Imports System.Collections.Concurrent
+
+Namespace ParallelExample
+    Class Program
+        Shared Sub Main()
+            ' 2 million
+            Dim limit = 2_000_000
+            Dim numbers = Enumerable.Range(0, limit).ToList()
+
+            Dim watch = Stopwatch.StartNew()
+            Dim primeNumbersFromForeach = GetPrimeList(numbers)
+            watch.Stop()
+
+            Dim watchForParallel = Stopwatch.StartNew()
+            Dim primeNumbersFromParallelForeach = GetPrimeListWithParallel(numbers)
+            watchForParallel.Stop()
+
+            Console.WriteLine($"Classical foreach loop | Total prime numbers : {primeNumbersFromForeach.Count} | Time Taken : {watch.ElapsedMilliseconds} ms.")
+            Console.WriteLine($"Parallel.ForEach loop  | Total prime numbers : {primeNumbersFromParallelForeach.Count} | Time Taken : {watchForParallel.ElapsedMilliseconds} ms.")
+
+            Console.WriteLine("Press any key to exit.")
+            Console.ReadLine()
+        End Sub
+
+        ' GetPrimeList returns Prime numbers by using sequential ForEach
+        Private Shared Function GetPrimeList(numbers As IList(Of Integer)) As IList(Of Integer)
+            Return numbers.Where(AddressOf IsPrime).ToList()
+        End Function
+
+        ' GetPrimeListWithParallel returns Prime numbers by using Parallel.ForEach
+        Private Shared Function GetPrimeListWithParallel(numbers As IList(Of Integer)) As IList(Of Integer)
+            Dim primeNumbers = New ConcurrentBag(Of Integer)()
+            Parallel.ForEach(numbers, Sub(number)
+
+                                          If IsPrime(number) Then
+                                              primeNumbers.Add(number)
+                                          End If
+                                      End Sub)
+            Return primeNumbers.ToList()
+        End Function
+
+        ' IsPrime returns true if number is Prime, else false.(https://en.wikipedia.org/wiki/Prime_number)
+        Private Shared Function IsPrime(number As Integer) As Boolean
+            If number < 2 Then
+                Return False
+            End If
+
+            For divisor = 2 To Math.Sqrt(number)
+
+                If number Mod divisor = 0 Then
+                    Return False
+                End If
+            Next
+
+            Return True
+        End Function
+    End Class
+End Namespace
+'</snippet03>

samples/snippets/visualbasic/VS_Snippets_Misc/tpl_parallel/vb/simpleforeach.vbproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net5.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>