如何：编写自定义 PLINQ 聚合函数

更新：2010 年 5 月

此示例演示如何使用 Aggregate 方法将自定义聚合函数应用于源序列。

警告
本示例旨在演示用法，运行速度可能不如等效的顺序 LINQ to Objects 查询快。有关加速的更多信息，请参见了解 PLINQ 中的加速。

示例

下面的示例计算一个整数序列的标准偏差。

Class aggregation
    Private Shared Sub Main(ByVal args As String())

        ' Create a data source for demonstration purposes.
        Dim source As Integer() = New Integer(99999) {}
        Dim rand As New Random()
        For x As Integer = 0 To source.Length - 1
            ' Should result in a mean of approximately 15.0.
            source(x) = rand.[Next](10, 20)
        Next

        ' Standard deviation calculation requires that we first
        ' calculate the mean average. Average is a predefined
        ' aggregation operator, along with Max, Min and Count.
        Dim mean As Double = source.AsParallel().Average()


        ' We use the overload that is unique to ParallelEnumerable. The 
        ' third Func parameter combines the results from each thread.
        ' initialize subtotal. Use decimal point to tell 
        ' the compiler this is a type double. Can also use: 0d.

        ' do this on each thread

        ' aggregate results after all threads are done.

        ' perform standard deviation calc on the aggregated result.
        Dim standardDev As Double = source.AsParallel().Aggregate(0.0R, Function(subtotal, item) subtotal + Math.Pow((item - mean), 2), Function(total, thisThread) total + thisThread, Function(finalSum) Math.Sqrt((finalSum / (source.Length - 1))))
        Console.WriteLine("Mean value is = {0}", mean)
        Console.WriteLine("Standard deviation is {0}", standardDev)

        Console.ReadLine()
    End Sub
End Class

namespace PLINQAggregation
{
    using System;
    using System.Linq;

    class aggregation
    {
        static void Main(string[] args)
        {

            // Create a data source for demonstration purposes.
            int[] source = new int[100000];
            Random rand = new Random();
            for (int x = 0; x < source.Length; x++)
            {
                // Should result in a mean of approximately 15.0.
                source[x] = rand.Next(10, 20);
            }

            // Standard deviation calculation requires that we first
            // calculate the mean average. Average is a predefined
            // aggregation operator, along with Max, Min and Count.
            double mean = source.AsParallel().Average();


            // We use the overload that is unique to ParallelEnumerable. The 
            // third Func parameter combines the results from each thread.
            double standardDev = source.AsParallel().Aggregate(
                // initialize subtotal. Use decimal point to tell 
                // the compiler this is a type double. Can also use: 0d.
                0.0,

                // do this on each thread
                 (subtotal, item) => subtotal + Math.Pow((item - mean), 2),

                 // aggregate results after all threads are done.
                 (total, thisThread) => total + thisThread,

                // perform standard deviation calc on the aggregated result.
                (finalSum) => Math.Sqrt((finalSum / (source.Length - 1)))
            );
            Console.WriteLine("Mean value is = {0}", mean);
            Console.WriteLine("Standard deviation is {0}", standardDev);
            Console.ReadLine();

        }
    }
}

此示例使用 Aggregate 标准查询运算符的重载，该运算符是 PLINQ 所独有的。 此重载采用额外的 System.Func<T1, T2, TResult> 作为第三个输入参数。 此委托在其对聚合的结果执行最终计算之前合并来自所有线程的结果。 在此示例中，我们将所有线程的和加总。

请注意，当 lambda 表达式体中只有一个表达式时，System.Func<T, TResult> 委托的返回值为该表达式的值。

请参见

参考

ParallelEnumerable

概念

并行 LINQ (PLINQ)

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