Linq扩展方法

如果要扩展LINQ查询方法集，只需要为IEnumerable<T>扩展方法。

第一种：扩展聚合方法，类似已有的Max、Min，可以给具体类型扩展，也可以给泛型扩展。

using System;using System.Collections;using System.Collections.Generic;using System.Data;using System.IO;using System.Linq;namespace ConsoleApp4{    class Program    {        static void Main(string[] args)        {            var numbers1 = new double[]{ 1.9, 2, 8, 4, 5.7, 6, 7.2, 0 };            var query1 = numbers1.Median();            Console.WriteLine("double: Median = " + query1);                        var numbers2 = new int[] { 4, 5, 6, 8, 0 };            var query2 = numbers2.Median();            Console.WriteLine($"int:Median ={query2}");            var str1 = new string[] { "ab", "abc", "abcde", "abcedfs" }; ;            var query3 = str1.Median(x => x.Length);            Console.WriteLine($"string:{query3}");        }         }    public static class LINQExtension    {        public static double Median(this IEnumerable<double> source)        {            if(source.Count()==0)            {                throw new InvalidOperationException("无法计算中位数");            }            var sortedList = source.OrderBy(x => x);            int itemIndex = (int)sortedList.Count()/2;            // 索引从0开始，itemIndex总是偏大            if(sortedList.Count()%2==0)            {                return (sortedList.ElementAt(itemIndex) + sortedList.ElementAt(itemIndex - 1)) / 2;            }            else            {                return sortedList.ElementAt(itemIndex);            }        }               //Int类型重载求中位数        public static double Median(this IEnumerable<int> source)        {                        return source.Select(x => Convert.ToDouble(x)).Median();                   }        //泛型，需传入选择器        public static double Median<T>(this IEnumerable<T> source,Func<T,double> selector)        {            return source.Select(x => selector(x)).Median();        }           }    }

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第二中：扩展返回集合的方法，类似Where、Orderby

public static class LINQExtension    {        public static IEnumerable<T> AlternateElements<T>(this IEnumerable<T> source)        {            List<T> list = new List<T>();            int i = 0;            foreach (var item in source)            {                if (i % 4 == 2)                {                    list.Add(item);                }            }            return list;                   }    }