Linq实现SQL中的递归查询

废话不多说，直接上代码：

using System;
using System.Collections.Generic;
using System.ComponentModel.Design;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Region> list = GetTestData();
            List<Region> query = GetResult(list, 2);
            foreach (Region region in query)
            {
                Console.WriteLine(region.Name);
            }
            Console.ReadKey();
        }

        // 测试数据
        static List<Region> GetTestData()
        {
            List<Region> list = new List<Region>();
            list.Add(new Region() { Id = 1, ParentId = 0, Name = "中国" });
            list.Add(new Region() { Id = 2, ParentId = 1, Name = "浙江" });
            list.Add(new Region() { Id = 3, ParentId = 1, Name = "江苏" });
            list.Add(new Region() { Id = 4, ParentId = 2, Name = "杭州" });
            list.Add(new Region() { Id = 5, ParentId = 2, Name = "宁波" });
            list.Add(new Region() { Id = 6, ParentId = 3, Name = "南京" });
            list.Add(new Region() { Id = 7, ParentId = 3, Name = "苏州" });
            return list;
        }

        // 递归查询
        static IEnumerable<Region> GetRegion(List<Region> list, int id)
        {
            IEnumerable<Region> query = from region in list
                                        where region.ParentId == id
                                        select region;
            return query.ToList().Concat(query.ToList().SelectMany(r => GetRegion(list, r.Id)));
        }

        // 生成最终结果
        static List<Region> GetResult(List<Region> list, int id)
        {
            List<Region> regions = new List<Region>();
            regions.AddRange(list.Where(r => r.Id == id));
            regions.AddRange(GetRegion(list, id));
            return regions;
        }
    }
}

运行结果如下：

浙江
杭州
宁波