using System;

using System.Collections;

using System.Collections.Generic;

using System.Reflection;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var categories = new List<Category>(){

            new(){ Id = 1 , Name="Sports",       ParentId = 0},

            new(){ Id = 2 , Name="Balls",        ParentId = 1},

            new(){ Id = 3 , Name="Shoes",        ParentId = 1},

            new(){ Id = 4 , Name="Electronics",  ParentId = 0},

            new(){ Id = 5 , Name="Cameras",      ParentId = 4},

            new(){ Id = 6 , Name="Lenses",       ParentId = 5},

            new(){ Id = 7 , Name="Tripod",       ParentId = 5},

            new(){ Id = 8 , Name="Computers",    ParentId = 4},

            new(){ Id = 9 , Name="Laptops",      ParentId = 8},

            new(){ Id = 10, Name="Empty",        ParentId = 0}

        };

        var root = GenericHelpers.GenerateTree(categories, c=> c.Id, c=> c.ParentId, (c, newItems) => c.Children = newItems);

        Test(root);

    }

    static void Test(IEnumerable<Category> categories, int deep = 0)

    {

        foreach (var c in categories)

        {

            Console.WriteLine(new String('\t', deep) + c.Name);

            Test(c.Children, deep + 1);

        }

    }

​

}

​

class Category

{

    public int Id;

    public int ParentId;

    public string Name;

    public IEnumerable<Category> Children;

}

​

internal static class GenericHelpers

{

    public static IEnumerable<T> GenerateTree<T, K>(

        this IEnumerable<T> collection,

        Func<T, K> id_selector,

        Func<T, K> parent_id_selector,

        Action<T, IEnumerable<T>> childrenApplier,

        K root_id = default(K))

    {

        foreach (var c in collection.Where(c => EqualityComparer<K>.Default.Equals(parent_id_selector(c), root_id)))

        {

            var children = collection.GenerateTree<T,K>(id_selector, parent_id_selector, childrenApplier, id_selector(c));

            childrenApplier(c, children);

            yield return c;

        }

    }

}