PLINQ 和 TPL 中的 Lambda 運算式
工作平行程式庫 (TPL) 包含了許多會採用其中一個委派的 System.Func<TResult> 或 System.Action 系列作為輸入參數的方法。 您可以使用這些委派將您自訂的程式邏輯傳遞到平行迴圈、工作或查詢中。 TPL 以及 PLINQ 的程式碼範例會使用 lambda 運算式來建立這些委派的執行個體，以作為內嵌程式碼區塊。 本主題將簡單介紹 Func 與 Action，並說明如何在工作平行程式庫和 PLINQ 中使用 lambda 運算式。
注意
一般來說，如需委派的詳細資訊，請參閱委派 (C#) 和委派 (Visual Basic)。 如需 C# 和 Visual Basic 中之 lambda 運算式的詳細資訊，請參閱 Lambda 運算式和 Lambda 運算式。
Func 委派
Func 委派所封裝的方法會傳回值。 在 Func 簽章中，最後一個或最右邊的型別參數一律會指定傳回型別。 其中一個常見的編譯器錯誤原因是嘗試將兩個輸入參數傳入 System.Func<T,TResult>；事實上，此型別只會採用一個輸入參數。 .NET 定義 17 個版本的 Func: System.Func<TResult>、System.Func<T,TResult>、System.Func<T1,T2,TResult> 等，依此類推一直到 System.Func<T1,T2,T3,T4,T5,T6,T7,T8,T9,T10,T11,T12,T13,T14,T15,T16,TResult>。
Action 委派
System.Action 委派所封裝的方法 (在 Visual Basic 中為 Sub) 不會傳回值。 在 Action 型別簽章中，型別參數只代表輸入參數。 就像 Func，.NET 定義 17 個版本的 Action，從不具型別參數的版本，一直到具有 16 個型別參數的版本。
範例
下列 Parallel.ForEach<TSource,TLocal>(IEnumerable<TSource>, Func<TLocal>, Func<TSource,ParallelLoopState,TLocal,TLocal>, Action<TLocal>) 方法的範例顯示如何透過使用 Lambda 運算式來表達 Func 與 Action 委派。
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
