PLINQ 和 TPL 中的 Lambda 表达式
任务并行库 (TPL) 包含许多方法，需要使用 System.Func<TResult> 或 System.Action 系列委托之一作为输入参数。 使用这些委托将自定义程序逻辑传入到并行循环、任务或查询中。 TPL 以及 PLINQ 的代码示例使用 lambda 表达式以内联代码块的形式创建这些委托的实例。 本主题简要介绍 Func 和 Action，并演示如何在任务并行库和 PLINQ 中使用 lambda 表达式。
注意
有关委托的更多常规信息，请参阅委托和委托。 有关 C# 和 Visual Basic 中的 lambda 表达式的详细信息，请参阅 Lambda 表达式和 Lambda 表达式。
Func 委托
Func 委托封装返回值的一个方法。 在 Func 签名中，最后或最右侧的类型参数始终指定返回类型。 导致编译器错误出现的常见原因之一是，尝试将两个输入参数传入 System.Func<T,TResult>；此类型其实只需要使用一个输入参数。 .NET 定义 Func 的 17 个版本：System.Func<TResult>、System.Func<T,TResult>、System.Func<T1,T2,TResult>，依此类推直到 System.Func<T1,T2,T3,T4,T5,T6,T7,T8,T9,T10,T11,T12,T13,T14,T15,T16,TResult>。
Action 委托
System.Action 委托封装了不返回值的方法（Visual Basic 中的 Sub）。 在 Action 类型签名中，类型参数仅表示输入参数。 与 Func 一样，.NET 定义了 Action 的 17 个版本，从没有类型参数的版本到具有 16 个类型参数的版本。
示例
下面的 Parallel.ForEach<TSource,TLocal>(IEnumerable<TSource>, Func<TLocal>, Func<TSource,ParallelLoopState,TLocal,TLocal>, Action<TLocal>) 方法示例展示了如何使用 Lambda 表达式表达 Func 和 Action 委托。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
class ForEachWithThreadLocal
{
// Demonstrated features:
// Parallel.ForEach()
// Thread-local state
// Expected results:
// This example sums up the elements of an int[] in parallel.
// Each thread maintains a local sum. When a thread is initialized, that local sum is set to 0.
// On every iteration the current element is added to the local sum.
// When a thread is done, it safely adds its local sum to the global sum.
// After the loop is complete, the global sum is printed out.
// Documentation:
// http://msdn.microsoft.com/library/dd990270(VS.100).aspx
static void Main()
{
// The sum of these elements is 40.
int[] input = { 4, 1, 6, 2, 9, 5, 10, 3 };
int sum = 0;
try
{
Parallel.ForEach(
input, // source collection
() => 0, // thread local initializer
(n, loopState, localSum) => // body
{
localSum += n;
Console.WriteLine("Thread={0}, n={1}, localSum={2}", Thread.CurrentThread.ManagedThreadId, n, localSum);
return localSum;
},
(localSum) => Interlocked.Add(ref sum, localSum) // thread local aggregator
);
Console.WriteLine("\nSum={0}", sum);
}
// No exception is expected in this example, but if one is still thrown from a task,
// it will be wrapped in AggregateException and propagated to the main thread.
catch (AggregateException e)
{
Console.WriteLine("Parallel.ForEach has thrown an exception. THIS WAS NOT EXPECTED.\n{0}", e);
}
}
}
Imports System.Threading
Imports System.Threading.Tasks
Module ForEachDemo
' Demonstrated features:
' Parallel.ForEach()
' Thread-local state
' Expected results:
' This example sums up the elements of an int[] in parallel.
' Each thread maintains a local sum. When a thread is initialized, that local sum is set to 0.
' On every iteration the current element is added to the local sum.
' When a thread is done, it safely adds its local sum to the global sum.
' After the loop is complete, the global sum is printed out.
' Documentation:
' http://msdn.microsoft.com/library/dd990270(VS.100).aspx
Private Sub ForEachDemo()
' The sum of these elements is 40.
Dim input As Integer() = {4, 1, 6, 2, 9, 5, _
10, 3}
Dim sum As Integer = 0
Try
' source collection
Parallel.ForEach(input,
Function()
' thread local initializer
Return 0
End Function,
Function(n, loopState, localSum)
' body
localSum += n
Console.WriteLine("Thread={0}, n={1}, localSum={2}", Thread.CurrentThread.ManagedThreadId, n, localSum)
Return localSum
End Function,
Sub(localSum)
' thread local aggregator
Interlocked.Add(sum, localSum)
End Sub)
Console.WriteLine(vbLf & "Sum={0}", sum)
Catch e As AggregateException
' No exception is expected in this example, but if one is still thrown from a task,
' it will be wrapped in AggregateException and propagated to the main thread.
Console.WriteLine("Parallel.ForEach has thrown an exception. THIS WAS NOT EXPECTED." & vbLf & "{0}", e)
End Try
End Sub
End Module
