Why sometimes 2 objects reference the same but not

问题:

Following the last answer : Recursive method to convert flat collection to hierarchal collection?

I want to use the same method CreateTree but with another object than Hierarchy: ItemNode:

public class ItemNode
{
    public string Id { get; set; }
    public Item Item { get; set; }
    public ICollection<ItemNode> Children { get; set; }
}

and the definition of Item:

public class Item
{
    public string ID { get; set; }
    public string Name { get; set; }
    public int Level { get; set; }
    public string ParentId { get; set; }
}

And here the CreateTree methods with the ItemNode:

    static List<ItemNode> CreateTreeItems(IEnumerable<ItemNode> nodes)
    {
        Dictionary<string,ItemNode> idToNode = nodes.ToDictionary(n => n.Id, n => n);
        List<ItemNode> roots = new List<ItemNode>();
        ItemNode root = null;

        foreach (var n in nodes)
        {
            if (n.Item.ParentId == null)
            {
                if (root != null)
                {
                    roots.Add(root);
                }
                root = n;
                continue;
            }

            ItemNode parent = idToNode[n.Item.ParentId];
            //if (!idToNode.TryGetValue(n.Item.ParentId, out parent))
            //{
            //  //Parent doesn't exist, orphaned entry
            //}

            parent?.Children.Add(n);

            // RETURNS FALSE WHEREAS IN THE ORIGINAL METHOD IT RETURNS TRUE
            var test = Object.ReferenceEquals(parent, root);
            Debug.WriteLine(test);
        }


        if (root == null)
        {
            //There was no root element
        }

        roots.Add(root);
        return roots;
    }

It does not work because parent and root does not reference the same object (whereas in the original method, it does). I guess it was linked to the fact that I have added an Item property to the ItemNode class. But I don't know how to fix it.

Thank you !

回答1:

Why do you expect the root and parent node references to be equal? Every root node might be a parent node but not every parent node is a root node.

There might be a situations where the references are equal but it very depends on a sort order of the nodes collection. Actually when root node is placed earlier than the 1st level child node you will see the situation where the references are equal.

I suppose your problem is elsewhere, for instance it might not work if the Nodes collection doesn't have a root node at all.

Here's the example, try it here https://dotnetfiddle.net/4r52xP

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

public class Hierarchy
{
    public Hierarchy(string iD, string name, int level, string parentID, string topParent)
    {
        ID = iD;
        Name = name;
        Level = level;
        ParentID = parentID;
        Children = new HashSet<Hierarchy>();
    }
    public string ID { get; set; }
    public string Name{ get; set; }
    public int Level { get; set; }
    public string ParentID { get; set; }
    public ICollection<Hierarchy> Children { get; set; }
}

public class Program
{
    static Hierarchy CreateTree(IEnumerable<Hierarchy> Nodes)
    {
        var idToNode = Nodes.ToDictionary(n => n.ID, n => n);

        Hierarchy root = null;
        foreach (var n in Nodes)
        {
            if (n.ParentID == null)
            {
                if (root != null)
                {
                    //there are multiple roots in the data
                }
                root = n;
                continue;
            }

            Hierarchy parent = null;
            if (!idToNode.TryGetValue(n.ParentID, out parent))
            {
                //Parent doesn't exist, orphaned entry
            }

            parent.Children.Add(n);

            Console.WriteLine("ReferenceEquals: {0}", Object.ReferenceEquals(parent, root));
        }

        if (root == null)
        {
            //There was no root element
        }
        return root;
    }

    public static void Main()
    {
        Console.WriteLine("Test #1");

        List<Hierarchy> l = new List<Hierarchy>();
        l.Add(new Hierarchy("295152","name1",1,null, null)); // <-root node at the top of the list
        l.Add(new Hierarchy("12345","child1",2,"295152", null));
        l.Add(new Hierarchy("54321","child2",2,"295152", null));
        l.Add(new Hierarchy("44444","child1a",3,"12345", null));
        l.Add(new Hierarchy("33333","child1b",3,"12345", null));
        l.Add(new Hierarchy("22222","child2a",3,"54321", null));
        l.Add(new Hierarchy("22221","child2b",3,"54321", null));
        l.Add(new Hierarchy("22002","child2c",3,"54321", null));
        l.Add(new Hierarchy("20001","child2a2",4,"22222", null));
        l.Add(new Hierarchy("20101","child2b2",4,"22222", null));       

        CreateTree(l);

        Console.WriteLine("\nTest #2");

        l = new List<Hierarchy>();
        l.Add(new Hierarchy("12345","child1",2,"295152", null));
        l.Add(new Hierarchy("54321","child2",2,"295152", null));
        l.Add(new Hierarchy("44444","child1a",3,"12345", null));
        l.Add(new Hierarchy("33333","child1b",3,"12345", null));
        l.Add(new Hierarchy("22222","child2a",3,"54321", null));
        l.Add(new Hierarchy("22221","child2b",3,"54321", null));
        l.Add(new Hierarchy("22002","child2c",3,"54321", null));
        l.Add(new Hierarchy("20001","child2a2",4,"22222", null));
        l.Add(new Hierarchy("20101","child2b2",4,"22222", null));
        l.Add(new Hierarchy("295152","name1",1,null, null)); // <-root node at the bottom of the list
        CreateTree(l);
    }
}