C#List<T> 扩展方法

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

namespace ListExtDemo
{
    // 集合扩展静态类
    public static class ListExtensions
    {
        public static bool IsNullOrEmpty<T>(this List<T> source)
        {
            return source == null || source.Count == 0;
        }

        public static List<T> DistinctSelf<T>(this List<T> source)
        {
            return source.IsNullOrEmpty() ? new List<T>() : source.Distinct().ToList();
        }

        public static List<T> Page<T>(this List<T> source, int pageIndex, int pageSize)
        {
            if (source.IsNullOrEmpty() || pageIndex < 1 || pageSize < 1)
                return new List<T>();
            return source.Skip((pageIndex - 1) * pageSize).Take(pageSize).ToList();
        }

        public static decimal GetSum<T>(this List<T> source, Func<T, decimal> selector)
        {
            return source.IsNullOrEmpty() ? 0m : source.Sum(selector);
        }

        public static Dictionary<TKey, TValue> ToMap<T, TKey, TValue>(
            this List<T> source, Func<T, TKey> keyFunc, Func<T, TValue> valFunc)
        {
            return source.IsNullOrEmpty()
                ? new Dictionary<TKey, TValue>()
                : source.ToDictionary(keyFunc, valFunc);
        }
    }

    // 测试实体类
    public class Goods
    {
        public int Id { get; set; }
        public string GoodsName { get; set; }
        public decimal Price { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<int> numList = new List<int> { 2, 5, 2, 8, 5, 9 };
            Console.WriteLine("====数值集合测试====");
            Console.WriteLine($"集合是否为空：{numList.IsNullOrEmpty()}");

            var distinctNum = numList.DistinctSelf();
            Console.Write("去重后数据：");
            distinctNum.ForEach(x => Console.Write($"{x} "));
            Console.WriteLine();

            List<Goods> goodsList = new List<Goods>
            {
                new Goods{Id=1,GoodsName="矿泉水",Price=2.5m},
                new Goods{Id=2,GoodsName="面包",Price=6.8m},
                new Goods{Id=3,GoodsName="牛奶",Price=4.2m},
                new Goods{Id=4,GoodsName="饼干",Price=8.9m}
            };

            Console.WriteLine("\n====实体集合测试====");
            var pageData = goodsList.Page(2, 2);
            Console.WriteLine("第二页分页数据：");
            foreach (var item in pageData)
            {
                Console.WriteLine($"编号：{item.Id} 名称：{item.GoodsName}");
            }

            decimal totalPrice = goodsList.GetSum(g => g.Price);
            Console.WriteLine($"商品总价：{totalPrice}");

            var goodsDict = goodsList.ToMap(g => g.Id, g => g.GoodsName);
            Console.WriteLine($"字典取值Id=2：{goodsDict[2]}");

            Console.ReadLine();
        }
    }
}

扩展方法必须和调用代码同命名空间，或者手动using 命名空间名引入，统一包裹后即可正常识别扩展方法。