C#FindAll VS Where Speed

Question

任何人都知道列表中Where和FindAll之间的任何速度差异.我知道IEnumerable的一部分在哪里,FindAll是List的一部分,我只是好奇什么更快.

Answer 1

List <T>类的FindAll方法实际构造一个新的列表对象,并向其添加结果.IEnumerable <T>的Where扩展方法将简单地迭代现有列表并生成匹配结果的枚举,而无需创建或添加任何内容(枚举器本身除外).

鉴于一小部分,两者可能表现相当.但是,给定一个更大的集合,Where应该优于FindAll,因为为包含结果而创建的新List必须动态增长以包含其他结果.随着匹配结果数量的增加,FindAll的内存使用量也将开始呈指数级增长,其中Where应该具有恒定的最小内存使用量(在其自身内部......排除您对结果所做的任何操作.)

Answer 2

FindAll显然比Where慢,因为它需要创建一个新列表.

无论如何,我认为你真的应该考虑Jon Hanna的评论 - 你可能需要对你的结果做一些操作,在很多情况下列表比IEnumerable更有用.

我写了一个小测试,只需将其粘贴到Console App项目中.它测量时间/滴答:函数执行,结果收集操作(获得'真实'用法的执行,并确保编译器不会优化未使用的数据等 - 我是C#的新手而不是知道它是如何工作的,对不起).

注意:除WhereIENumerable()之外的每个测量函数都会创建新的元素列表.我可能做错了什么,但显然迭代IEnumerable比迭代列表需要更多的时间.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;

namespace Tests
{

    public class Dummy
    {
        public int Val;
        public Dummy(int val)
        {
            Val = val;
        }
    }
    public class WhereOrFindAll
    {
        const int ElCount = 20000000;
        const int FilterVal =1000;
        const int MaxVal = 2000;
        const bool CheckSum = true; // Checks sum of elements in list of resutls
        static List<Dummy> list = new List<Dummy>();
        public delegate void FuncToTest();

        public static long TestTicks(FuncToTest function, string msg)
        {
            Stopwatch watch = new Stopwatch();
            watch.Start();
            function();
            watch.Stop();
            Console.Write("\r\n"+msg + "\t ticks: " + (watch.ElapsedTicks));
            return watch.ElapsedTicks;
        }
        static void Check(List<Dummy> list)
        {
            if (!CheckSum) return;
            Stopwatch watch = new Stopwatch();
            watch.Start();

            long res=0;
            int count = list.Count;
            for (int i = 0; i < count; i++)     res += list[i].Val;
            for (int i = 0; i < count; i++)     res -= (long)(list[i].Val * 0.3);

            watch.Stop();
            Console.Write("\r\n\nCheck sum: " + res.ToString() + "\t iteration ticks: " + watch.ElapsedTicks);
        }
        static void Check(IEnumerable<Dummy> ieNumerable)
        {
            if (!CheckSum) return;
            Stopwatch watch = new Stopwatch();
            watch.Start();

            IEnumerator<Dummy> ieNumerator = ieNumerable.GetEnumerator();
            long res = 0;
            while (ieNumerator.MoveNext())  res += ieNumerator.Current.Val;
            ieNumerator=ieNumerable.GetEnumerator();
            while (ieNumerator.MoveNext())  res -= (long)(ieNumerator.Current.Val * 0.3);

            watch.Stop();
            Console.Write("\r\n\nCheck sum: " + res.ToString() + "\t iteration ticks :" + watch.ElapsedTicks);
        }
        static void Generate()
        {
            if (list.Count > 0)
                return;
            var rand = new Random();
            for (int i = 0; i < ElCount; i++)
                list.Add(new Dummy(rand.Next(MaxVal)));

        }
        static void For()
        {
            List<Dummy> resList = new List<Dummy>();
            int count = list.Count;
            for (int i = 0; i < count; i++)
            {
                if (list[i].Val < FilterVal)
                    resList.Add(list[i]);
            }      
            Check(resList);
        }
        static void Foreach()
        {
            List<Dummy> resList = new List<Dummy>();
            int count = list.Count;
            foreach (Dummy dummy in list)
            {
                if (dummy.Val < FilterVal)
                    resList.Add(dummy);
            }
            Check(resList);
        }
        static void WhereToList()
        {
            List<Dummy> resList = list.Where(x => x.Val < FilterVal).ToList<Dummy>();
            Check(resList);
        }
        static void WhereIEnumerable()
        {
            Stopwatch watch = new Stopwatch();
            IEnumerable<Dummy> iEnumerable = list.Where(x => x.Val < FilterVal);
            Check(iEnumerable);
        }
        static void FindAll()
        {
            List<Dummy> resList = list.FindAll(x => x.Val < FilterVal);
            Check(resList);
        }
        public static void Run()
        {
            Generate();
            long[] ticks = { 0, 0, 0, 0, 0 };
            for (int i = 0; i < 10; i++)
            {
                ticks[0] += TestTicks(For, "For \t\t");
                ticks[1] += TestTicks(Foreach, "Foreach \t");
                ticks[2] += TestTicks(WhereToList, "Where to list \t");
                ticks[3] += TestTicks(WhereIEnumerable, "Where Ienum \t");
                ticks[4] += TestTicks(FindAll, "FindAll \t");
                Console.Write("\r\n---------------");
            }
            for (int i = 0; i < 5; i++)
                Console.Write("\r\n"+ticks[i].ToString());
        }

    }

    class Program
    {
        static void Main(string[] args)
        {
            WhereOrFindAll.Run();
            Console.Read();
        }
    }
}

结果(滴答) - 启用CheckSum(对结果执行某些操作),模式:不调试释放(CTRL + F5):

• 16222276(for - > list)
• 17151121(foreach - > list)
• 4741494(其中 - >列表)
• 27122285(其中 - > ienum)
• 18821571(findall - > list)

CheckSum已禁用(根本不使用返回的列表):

• 10885004(for - > list)
• 11221888(foreach - > list)
• 18688433(其中 - >列表)
• 1075(其中 - > ienum)
• 13720243(findall - > list)

您的结果可能略有不同,要获得真正的结果,您需要更多的迭代.