How to restrict Parallel.ForEach in C#?

Parallel Foreach

The Parallel.ForEach loop in C# runs on multiple threads and processing happens in parallel. Parallel.ForEach loops are not a basic feature of C# and are available starting with C# 4.0 and later. To use Parallel.ForEach loop, we need to import System.Threading.Tasks namespace in using directive.

Foreach h2>

The Foreach loop in C# runs on a single thread and processing occurs sequentially one by one. Foreach loops are a fundamental feature of C#, available starting with C# 1.0. In most cases, it performs slower than Parallel.Foreach.

Example 1

static void Main(string[] args){
   List<string> alphabets = new List<string>();
   alphabets.Add("A");
   alphabets.Add("B");
   alphabets.Add("C");
   alphabets.Add("D");
   alphabets.Add("E");
   alphabets.Add("F");
   alphabets.Add("G");
   alphabets.Add("H");
   alphabets.Add("I");
   alphabets.Add("J");
   alphabets.Add("K");
   alphabets.Add("L");
   alphabets.Add("M");
   alphabets.Add("N");
   alphabets.Add("O");
   alphabets.Add("P");
   alphabets.Add("Q");
   alphabets.Add("R");
   alphabets.Add("S");
   alphabets.Add("T");
   alphabets.Add("U");
   alphabets.Add("V");
   alphabets.Add("W");
   alphabets.Add("X");
   alphabets.Add("Y");
   alphabets.Add("Z");
   Console.WriteLine("Printing list using foreach loop");
   var stopWatch = Stopwatch.StartNew();
   foreach (string alphabet in alphabets){
      Console.WriteLine("alphabet Name: {0}, Thread Id= {1}", alphabet,          Thread.CurrentThread.ManagedThreadId);
   }
   Console.WriteLine("foreach loop execution time = {0} seconds", stopWatch.Elapsed.TotalSeconds);
   Console.WriteLine("Printing list using Parallel.ForEach");
   stopWatch = Stopwatch.StartNew();
   Parallel.ForEach(alphabets, alphabet =>
   {
      Console.WriteLine("alphabet Name: {0}, Thread Id= {1}", alphabet,       Thread.CurrentThread.ManagedThreadId);
   }
   );
   Console.WriteLine("Parallel.ForEach() execution time = {0} seconds",    stopWatch.Elapsed.TotalSeconds);
   Console.Read();
   Console.ReadLine();
}

Output

Use foreach loop to print the list

alphabet Name: A, Thread Id= 1
alphabet Name: B, Thread Id= 1
alphabet Name: C, Thread Id= 1
alphabet Name: D, Thread Id= 1
alphabet Name: E, Thread Id= 1
alphabet Name: F, Thread Id= 1
alphabet Name: G, Thread Id= 1
alphabet Name: H, Thread Id= 1
alphabet Name: I, Thread Id= 1
alphabet Name: J, Thread Id= 1
alphabet Name: K, Thread Id= 1
alphabet Name: L, Thread Id= 1
alphabet Name: M, Thread Id= 1
alphabet Name: N, Thread Id= 1
alphabet Name: O, Thread Id= 1
alphabet Name: P, Thread Id= 1
alphabet Name: Q, Thread Id= 1
alphabet Name: R, Thread Id= 1
alphabet Name: S, Thread Id= 1
alphabet Name: T, Thread Id= 1
alphabet Name: U, Thread Id= 1
alphabet Name: V, Thread Id= 1
alphabet Name: W, Thread Id= 1
alphabet Name: X, Thread Id= 1
alphabet Name: Y, Thread Id= 1
alphabet Name: Z, Thread Id= 1
foreach loop execution time = 0.0223421 seconds

Use Parallel.ForEach to print the list

alphabet Name: A, Thread Id= 1
alphabet Name: G, Thread Id= 4
alphabet Name: H, Thread Id= 4
alphabet Name: I, Thread Id= 4
alphabet Name: J, Thread Id= 4
alphabet Name: K, Thread Id= 4
alphabet Name: L, Thread Id= 4
alphabet Name: N, Thread Id= 4
alphabet Name: O, Thread Id= 4
alphabet Name: P, Thread Id= 4
alphabet Name: Q, Thread Id= 4
alphabet Name: Y, Thread Id= 6
alphabet Name: Z, Thread Id= 6
alphabet Name: D, Thread Id= 6
alphabet Name: E, Thread Id= 6
alphabet Name: F, Thread Id= 6
alphabet Name: T, Thread Id= 6
alphabet Name: U, Thread Id= 6
alphabet Name: V, Thread Id= 6
alphabet Name: R, Thread Id= 4
alphabet Name: M, Thread Id= 5
alphabet Name: S, Thread Id= 7
alphabet Name: B, Thread Id= 1
alphabet Name: C, Thread Id= 1
alphabet Name: W, Thread Id= 6
alphabet Name: X, Thread Id= 6
Parallel.ForEach() execution time = 0.0559777 seconds

Example 2

Limiting parallelism in parallel.foreach

static class Program{
   static void Main(string[] args){
      List<string> alphabets = new List<string>();
      alphabets.Add("A");
      alphabets.Add("B");
      alphabets.Add("C");
      alphabets.Add("D");
      alphabets.Add("E");
      alphabets.Add("F");
      alphabets.Add("G");
      alphabets.Add("H");
      alphabets.Add("I");
      alphabets.Add("J");
      alphabets.Add("K");
      alphabets.Add("L");
      alphabets.Add("M");
      alphabets.Add("N");
      alphabets.Add("O");
      alphabets.Add("P");
      alphabets.Add("Q");
      alphabets.Add("R");
      alphabets.Add("S");
      alphabets.Add("T");
      alphabets.Add("U");
      alphabets.Add("V");
      alphabets.Add("W");
      alphabets.Add("X");
      alphabets.Add("Y");
      alphabets.Add("Z");
      Parallel.ForEach(
      alphabets,
      new ParallelOptions { MaxDegreeOfParallelism = 2 },
      alphabet => { Console.WriteLine("alphabet Name: {0}, Thread Id= {1}", alphabet,       Thread.CurrentThread.ManagedThreadId); }
   );
   }
}

Output

alphabet Name: N, Thread Id= 4
alphabet Name: O, Thread Id= 4
alphabet Name: P, Thread Id= 4
alphabet Name: A, Thread Id= 1
alphabet Name: B, Thread Id= 1
alphabet Name: C, Thread Id= 1
alphabet Name: Q, Thread Id= 4
alphabet Name: R, Thread Id= 4
alphabet Name: S, Thread Id= 4
alphabet Name: T, Thread Id= 4
alphabet Name: U, Thread Id= 4
alphabet Name: V, Thread Id= 4
alphabet Name: W, Thread Id= 4
alphabet Name: X, Thread Id= 4
alphabet Name: Y, Thread Id= 4
alphabet Name: Z, Thread Id= 4
alphabet Name: H, Thread Id= 4
alphabet Name: I, Thread Id= 4
alphabet Name: J, Thread Id= 4
alphabet Name: K, Thread Id= 4
alphabet Name: D, Thread Id= 1
alphabet Name: L, Thread Id= 4
alphabet Name: E, Thread Id= 1
alphabet Name: F, Thread Id= 1
alphabet Name: G, Thread Id= 1
alphabet Name: M, Thread Id= 4

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