Linq擴展方法-有解無憂

如果要擴展LINQ查詢方法集，只需要為IEnumerable<T>擴展方法，

第一種：擴展聚合方法，類似已有的Max、Min，可以給具體型別擴展，也可以給泛型擴展，

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("無法計算中位數");
            }

            var sortedList = source.OrderBy(x => x);

            int itemIndex = (int)sortedList.Count()/2;

            // 索引從0開始，itemIndex總是偏大
            if(sortedList.Count()%2==0)
            {
                return (sortedList.ElementAt(itemIndex) + sortedList.ElementAt(itemIndex - 1)) / 2;
            }
            else
            {
                return sortedList.ElementAt(itemIndex);
            }
        }
       
        //Int型別多載求中位數
        public static double Median(this IEnumerable<int> source)
        {            
            return source.Select(x => Convert.ToDouble(x)).Median();           
        }

        //泛型，需傳入選擇器
        public static double Median<T>(this IEnumerable<T> source,Func<T,double> selector)
        {
            return source.Select(x => selector(x)).Median();
        }       
    }    
}

View Code

第二中：擴展回傳集合的方法，類似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;           
        }
    }

轉載請註明出處，本文鏈接：https://www.uj5u.com/net/64878.html

標籤：C#

上一篇：關于“Failed to complete setup of assembly(hr = 0x80131040). Probing terminated”

下一篇：事件的基本使用