Recursive Hierarchical Parent child

Question

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

Here is my class design:

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

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.

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

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

How do I achieve this?

Answer 1

You could use this approach:

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();

---

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

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