递归层次家长的孩子 [英] Recursive Hierarchical Parent child

问题描述

我有一个项目，从它有一个 PARENTID 值或空数据库未来的集合。

I have a collection of items coming from a database which has a parentid value or null.

下面是我的课的设计：

public class Item
{
public int id{get;set;}
public string Name{get;set;}
public int? ParentId{get;set;}
public List<Item> SubItems{get;set;}
}

我想建立从集合项目的层次结构。假设一个集合是100个项目从中我需要根据的ParentId映射构建结构。

I want to build a hierarchical structure of Items from the collection. Assume a collection is 100 items from which I need to construct the structure based on the ParentId mapping.

我想这个帖子<一个href="http://www.superstar$c$crs.com/blogs/posts/recursive-hierarchical-joins-in-c-sharp-and-linq.aspx"相对=nofollow>递归分层加入C＃和LINQ  但它给了我一个错误，如果的ParentId为null。

I tried this post Recursive Hierarchical Joins in C# and LINQ but it gives me an error if ParentId is null.

也试过<一个href="http://stackoverflow.com/questions/15867478/build-tree-type-list-by-recursively-checking-parent-child-relationship-c-sharp">Build通过递归检查父子关系的C＃，但这种方法也并没有为我工作树型列表。

Also tried Build tree type list by recursively checking parent-child relationship C# , but this solution also does not work for me.

我如何做到这一点？

推荐答案

您可以使用此方法：

1. 从数据库中获取的所有项目（不填充子项）。
2. 构建查找＆LT; INT，项目＆GT; 父ID和项目与父母的id
3. 遍历的项目和关联使用查找该子项的每个项目。

1. Get all the items from the database (without filling the SubItems).
2. Build a Lookup<int?,Item> of parent ids and items with that parent id.
3. Loop through the items and associate each item with the subitems using the lookup.

code：

var items = // get from the database... (e.g. as a list)
var lookup = items.ToLookup(x => x.ParentId);
foreach (var item in items)
    item.SubItems = lookup[item.Id].ToList();

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由于 @EamonNerbonne 下面的评论，你可以得到的根元素为好，如果你需要：

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As @EamonNerbonne commented below, you can get the root elements as well, if you need to:

var roots = lookup[null].ToList();

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