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如果LINQ Count()擴展方法上IEnumerable<T>具有Count屬性調用（如List<T>），請問Count()方法來查找該財產，並將其返回（而不是通過枚舉計數的項目他們）？下面的測試代碼似乎表明，它的作用：C＃伯爵（）擴展方法性能
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(); }
}
}
相關提示：怎麼能一個實現IEnumerable<T>自定義集合公開自己的Count財產以這樣的方式在CLR Count()擴展方法可以採取它的優點？