LINQ:遍历层次结构以检索父层次结构

Question

我有一些表示同一实体上的父子关系的数据。给定一个节点，我需要找到它的整个上层层次结构(父、祖父母、曾祖父母等)。

我的实体是这样的：

 public partial class Location{
        public int LocationId { get; set; }
        public int? FkParentLocationId { get; set; }
        ..... more properties here.......
        public virtual Location FkParentLocation { get; set; }
        public virtual ICollection<Location> InverseFkParentLocation { get; set; }
}

我指的是here建议的层次结构遍历实现，但它在向下层次结构时起作用。如何使用LINQ检索上层层次结构？

示例数据：

            List<Location> locations = new List<Location> {
            new Location { LocationId = 5, FkParentLocationId = 3, LocationName = "Windsor", LocationDisplayName = "Windsor"},
            new Location { LocationId = 15, FkParentLocationId = 3, LocationName = "Hampshire", LocationDisplayName = "Hampshire" },
            new Location { LocationId = 12, FkParentLocationId = 3, LocationName = "Sussex", LocationDisplayName = "Sussex"},
            new Location { LocationId = 13, FkParentLocationId = 3, LocationName = "Willowood", LocationDisplayName = "Willowood"},
            new Location { LocationId = 1, FkParentLocationId = 3, LocationName = "Gerbshire", LocationDisplayName = "Gerbshire"},
            new Location { LocationId = 3, FkParentLocationId = 2, LocationName = "Lincoln", LocationDisplayName = "Lincoln"},
            new Location { LocationId = 2, LocationName = "Mains", LocationDisplayName = "Mains" }            };

预期输出:给定位置Id:5，我应该会得到一个包含位置3和2的列表(因为它们是父级)。

Answer 1

这是一种你可以使用的方法，使用控制台应用程序进行演示。大量向Jon Skeet借钱。

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

namespace Locations
{
    public partial class Location 
    {
        public int LocationId { get; set; }
        public int? FkParentLocationId { get; set; }
        public virtual Location FkParentLocation { get; set; }
        public virtual ICollection<Location> InverseFkParentLocation { get; set; }
        public string LocationName { get; set; }
        public string LocationDisplayName { get; set; }      
    }  

    class Program
    {
        static void Main(string[] args)
        {
            List<Location> locations = new List<Location> {
            new Location { LocationId = 5, FkParentLocationId = 3, LocationName = "Windsor", LocationDisplayName = "Windsor"},
            new Location { LocationId = 15, FkParentLocationId = 3, LocationName = "Hampshire", LocationDisplayName = "Hampshire" },
            new Location { LocationId = 12, FkParentLocationId = 3, LocationName = "Sussex", LocationDisplayName = "Sussex"},
            new Location { LocationId = 13, FkParentLocationId = 3, LocationName = "Willowood", LocationDisplayName = "Willowood"},
            new Location { LocationId = 1, FkParentLocationId = 3, LocationName = "Gerbshire", LocationDisplayName = "Gerbshire"},
            new Location { LocationId = 3, FkParentLocationId = 2, LocationName = "Lincoln", LocationDisplayName = "Lincoln"},
            new Location { LocationId = 2, LocationName = "Mains", LocationDisplayName = "Mains" }            };


            var result  = GetAncestorsIds(locations, 5);
            foreach (var id in result)
            {
                System.Console.WriteLine(id);
            }          
        }

        private static IEnumerable<int> GetAncestorsIds(List<Location> locations, int id)
        {
            Location location = locations.SingleOrDefault(l => l.LocationId == id);
            if(location != null)
            {   
                while(location != null && location.FkParentLocationId != null)
                {
                    location = locations.SingleOrDefault(l => l.LocationId == location.FkParentLocationId);
                    if(location != null)
                    {
                        yield return location.LocationId;
                    }
                }
            }   
        }
    }
}

这种方法可以转化为您自己的Linq扩展。这是它可能看起来的样子。

public static class MyExtensions
{
    public static IEnumerable<int> GetAncestorIds<TSource>(this IEnumerable<TSource> source, Func<TSource, int> pk, Func<TSource, int?> fk, int id)
    {
        TSource currentObj = source.SingleOrDefault(s => pk(s) == id);

        while(currentObj != null && fk(currentObj) != null)
        {
            currentObj = source.SingleOrDefault(s => pk(s) == fk(currentObj));
            if(currentObj != null)
            {
                yield return pk(currentObj);
            }
        }
    }
}

然后在你的场景中调用它，你可以这样做

var result = locations.GetAncestorIds(l => l.LocationId, l => l.FkParentLocationId, 5);