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          


        

Answer 1

You could use this approach:

1. Get all the items from the database (without filling the SubItems).
2. Build a Lookup 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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As @EamonNerbonne commented below, you can get the root elements as well, if you need to:

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