如何：在 PLINQ 中指定合并选项

项目
2023/04/06

此示例展示了如何指定应用于 PLINQ 查询中所有后续运算符的合并选项。虽然无需显式设置合并选项，但这样做可以提升性能。若要详细了解合并选项，请参阅 PLINQ 中的合并选项。

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。若要详细了解加速，请参阅了解 PLINQ 中的加速。

示例

下面的示例展示了合并选项在基本方案中的行为，此方案包含无序源，并将高成本函数应用于每个元素。

namespace MergeOptions
{
    using System;
    using System.Diagnostics;
    using System.Linq;
    using System.Threading;

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

            var nums = Enumerable.Range(1, 10000);

            // Replace NotBuffered with AutoBuffered
            // or FullyBuffered to compare behavior.
            var scanLines = from n in nums.AsParallel()
                                .WithMergeOptions(ParallelMergeOptions.NotBuffered)
                            where n % 2 == 0
                            select ExpensiveFunc(n);

            Stopwatch sw = Stopwatch.StartNew();
            foreach (var line in scanLines)
            {
                Console.WriteLine(line);
            }

            Console.WriteLine("Elapsed time: {0} ms. Press any key to exit.",
                            sw.ElapsedMilliseconds);
            Console.ReadKey();
        }

        // A function that demonstrates what a fly
        // sees when it watches television :-)
        static string ExpensiveFunc(int i)
        {
            Thread.SpinWait(2000000);
            return string.Format("{0} *****************************************", i);
        }
    }
}

Class MergeOptions2
    Sub DoMergeOptions()

        Dim nums = Enumerable.Range(1, 10000)

        ' Replace NotBuffered with AutoBuffered
        ' or FullyBuffered to compare behavior.
        Dim scanLines = From n In nums.AsParallel().WithMergeOptions(ParallelMergeOptions.NotBuffered)
                        Where n Mod 2 = 0
                        Select ExpensiveFunc(n)

        Dim sw = Stopwatch.StartNew()
        For Each line In scanLines
            Console.WriteLine(line)
        Next

        Console.WriteLine("Elapsed time: {0} ms. Press any key to exit.")
        Console.ReadKey()

    End Sub
    ' A function that demonstrates what a fly
    ' sees when it watches television :-)
    Function ExpensiveFunc(ByVal i As Integer) As String
        Threading.Thread.SpinWait(2000000)
        Return String.Format("{0} *****************************************", i)
    End Function
End Class

如果 AutoBuffered 选项在第一个元素生成前导致不利延迟发生，请尝试使用 NotBuffered 选项，更快更顺畅地生成结果元素。