Tree.cs

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

namespace explainpowershell.frontend
{
    public class TreeItem<T>
    {
        public T Item { get; set; }
        public HashSet<TreeItem<T>> Children { get; set; }

        public bool IsExpanded {get; set;} = true;
        public bool HasChildren {
            get {
                return Children.Count > 0;
            }
        }
    }

    internal static class GenericTree
    {
        /// <summary>
        /// Generates tree of items from item list
        /// </summary>
        /// 
        /// <typeparam name="T">Type of item in collection</typeparam>
        /// <typeparam name="K">Type of parent_id</typeparam>
        /// 
        /// <param name="collection">Collection of items</param>
        /// <param name="id_selector">Function extracting item's id</param>
        /// <param name="parent_id_selector">Function extracting item's parent_id</param>
        /// <param name="root_id">Root element id</param>
        /// 
        /// <returns>Tree of items</returns>
        public static HashSet<TreeItem<T>> GenerateTree<T, K>(
            this IEnumerable<T> collection,
            Func<T, K> id_selector,
            Func<T, K> parent_id_selector,
            K root_id = default)
        {
            var hashset = new HashSet<TreeItem<T>>();
            foreach (var c in collection.Where(c => EqualityComparer<K>.Default.Equals(parent_id_selector(c), root_id)))
            {
                hashset.Add(new TreeItem<T>
                {
                    Item = c,
                    Children = collection.GenerateTree(id_selector, parent_id_selector, id_selector(c))
                });
            }
            return hashset;
        }
    }
}