如何限制 C# 中的 Parallel.ForEach?

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并行 Foreach

C# 中的 Parallel.ForEach 循环在多个线程上运行,处理以并行方式进行。Parallel.ForEach 循环不是 C# 的基本功能,它从 C# 4.0 及更高版本开始可用。要使用 Parallel.ForEach 循环,我们需要在 using 指令中导入 System.Threading.Tasks 命名空间。

Foreach

C# 中的 Foreach 循环在单个线程上运行,处理按顺序逐个进行。Foreach 循环是 C# 的基本功能,从 C# 1.0 开始可用。在大多数情况下,它的执行速度比 Parallel.Foreach 慢。

示例 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(); }

输出

使用 foreach 循环打印列表

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

使用 Parallel.ForEach 打印列表

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

示例 2

限制 parallel.foreach 中的并行性

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); }
   );
   }
}

输出

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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