本示例显示如何使用 Parallel.ForEach 循环对任何 System.Collections.IEnumerable 或 System.Collections.Generic.IEnumerable<T> 数据源启用数据并行。
.gif "注意") 注意 |
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本文档使用 lambda 表达式在 PLINQ 中定义委托。如果您不熟悉 C# 或 Visual Basic 中的 lambda 表达式，请参见 在 PLINQ 和 TPL 中的 Lambda 表达式。 |
示例
' How to: Write a Simple Parallel.ForEach Loop
' IMPORTANT!!!: Add reference to System.Drawing.dll
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() = System.IO.Directory.GetFiles("C:\Users\Public\Pictures\Sample Pictures", "*.jpg")
Dim newDir As String = "C:\Users\Public\Pictures\Sample Pictures\Modified"
System.IO.Directory.CreateDirectory(newDir)
' Method signature: Parallel.ForEach(IEnumerable<TSource> source, Action<TSource> body)
' Be sure to add a reference to System.Drawing.dll.
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 = System.IO.Path.GetFileName(currentFile)
Dim bitmap As New System.Drawing.Bitmap(currentFile)
bitmap.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone)
bitmap.Save(System.IO.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 {0} on thread {1}", filename, 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
namespace ForEachDemo
{
using System;
using System.Drawing; // requires system.Drawing.dll
using System.IO;
using System.Threading;
using System.Threading.Tasks;
class SimpleForEach
{
static void Main()
{
// A simple source for demonstration purposes. Modify this path as necessary.
string[] files = System.IO.Directory.GetFiles(@"C:\Users\Public\Pictures\Sample Pictures", "*.jpg");
string newDir = @"C:\Users\Public\Pictures\Sample Pictures\Modified";
System.IO.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 = System.IO.Path.GetFileName(currentFile);
System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(currentFile);
bitmap.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);
bitmap.Save(System.IO.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 {0} on thread {1}", filename,
Thread.CurrentThread.ManagedThreadId);
} //close lambda expression
); //close method invocation
// Keep the console window open in debug mode.
Console.WriteLine("Processing complete. Press any key to exit.");
Console.ReadKey();
}
}
}
ForEach 循环的工作方式类似于 For 循环。 根据系统环境，对源集合进行分区，并在多个线程上计划工作。 系统中的处理器越多，并行方法的运行速度越快。 对于某些源集合，顺序循环可能更快，具体取决于源的大小和正在执行的工作类型。 有关性能的更多信息，请参见数据并行和任务并行中的潜在缺陷
有关并行循环的更多信息，请参见如何：编写简单的 Parallel.For 循环
若要将 ForEach 用于非泛型集合，可以使用 Cast<TResult> 扩展方法将集合转换为泛型集合，如下面的示例所示：
Parallel.ForEach(nonGenericCollection.Cast(Of Object), _
Sub(currentElement)
' ... work with currentElement
End Sub)
Parallel.ForEach(nonGenericCollection.Cast<object>(),
currentElement =>
{
});
还可以使用并行 LINQ (PLINQ) 来并行处理 IEnumerable<T> 数据源。 PLINQ 使您可以使用声明性的查询语法表达循环行为。 有关更多信息，请参见并行 LINQ (PLINQ)。
编译代码
将此代码复制并粘贴到 Visual Studio 2010 控制台应用程序项目中。
添加对 System.Drawing.dll 的引用
按 F5