C#Count()扩展方法性能

Question

如果在具有属性的Count()情况下调用LINQ 扩展方法(例如),该方法是否查找该属性并将其返回(而不是通过枚举它来计算项目)？以下测试代码似乎表明它确实:IEnumerable<T>CountList<T>Count()

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

namespace CountSpeedTest
{
    // Output:
    // List      - CLR : 0 ms
    // Enumerate - CLR : 10 ms
    // List      - Mine: 12 ms
    // Enumerate - Mine: 12 ms
    class Program
    {
        private const int Runs = 10;
        private const int Items = 1000000;

        static void Main(string[] args)
        {
            var total = new long[] {0, 0, 0, 0};
            for (int i = 0; i < Runs; ++i)
            {
                var items = Enumerable.Range(0, Items).Select(o => o.ToString()).ToList();
                var list = new List<string>(items);
                var enumerate = new Enumerate<string>(items);
                total[0] += TimeCount(list, c => c.Count());
                total[1] += TimeCount(enumerate, c => c.Count());
                total[2] += TimeCount(list, c => c.SlowCount());
                total[3] += TimeCount(enumerate, c => c.SlowCount());
            }
            Console.WriteLine(String.Format("List      - CLR : {0} ms", total[0] / Runs));
            Console.WriteLine(String.Format("Enumerate - CLR : {0} ms", total[1] / Runs));
            Console.WriteLine(String.Format("List      - Mine: {0} ms", total[2] / Runs));
            Console.WriteLine(String.Format("Enumerate - Mine: {0} ms", total[3] / Runs));
            Console.ReadKey(true);
        }

        private static long TimeCount<T>(IEnumerable<T> collection, Func<IEnumerable<T>, int> counter)
        {
            var stopwatch = Stopwatch.StartNew();
            var count = counter(collection);
            stopwatch.Stop();
            if (count != Items) throw new Exception("Incorrect Count");
            return stopwatch.ElapsedMilliseconds;
        }
    }

    public static class CountExtensions
    {
        // Performs a simple enumeration based count.
        public static int SlowCount<T>(this IEnumerable<T> items)
        {
            var i = 0;
            var enumerator = items.GetEnumerator();
            while (enumerator.MoveNext()) i++;
            return i;
        }
    }

    // Wraps an IEnumerable<T> to hide its Count property.
    public class Enumerate<T> : IEnumerable<T>
    {
        private readonly IEnumerable<T> collection;
        public Enumerate(IEnumerable<T> collection) { this.collection = collection; }

        public IEnumerator<T> GetEnumerator() { return collection.GetEnumerator(); }
        IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
    }
}

在相关的说明中:实现的自定义集合如何以CLR 扩展方法可以利用它的方式IEnumerable<T>公开其自己的Count属性Count()？

Answer 1

它不会Count按名称查找属性,但会检查它是否实现ICollection<T>,然后使用该类型的Count属性.从文档:

如果源类型实现 ICollection<T>,则使用该实现来获取元素的数量.否则,此方法确定计数.

(显然这只适用于不带谓词的重载.)

因此,如果您想有效地获得计数,请确保实施ICollection<T>.

Answer 2

是的,ICollection<T>如果找到,Enumerable.Count方法确实会查找并使用它的Count属性.您可以通过查看反射器中的Enumerable.Count来验证这一点.

仅当您使用不带其他参数的Count扩展方法时,才会出现这种情况.如果您使用带谓词的版本,它将遍历可枚举元素.