Create List Heirarchy from flat collection of entities

namespace TUEMIS.Common
{
    public static class TreeExtensions
    {
private static IEnumerable<TreeNode<TEntity>> CreateHierarchy<TEntity, TProperty>(IEnumerable<TEntity> items, TEntity parentItem,
            Func<TEntity, TProperty> idProperty, Func<TEntity, TProperty> parentIdProperty, int level, Func<TEntity, TProperty> orderBy) where TEntity : class
        {
            items = items.OrderBy(orderBy);

            IEnumerable<TEntity> childs;
            if (parentItem == null)
                childs = items.Where(i => parentIdProperty(i).Equals(default(TProperty)));
            else
                childs = items.Where(i => parentIdProperty(i).Equals(idProperty(parentItem)));

            if (childs.Any())
            {
                level++;
                foreach (var item in childs)
                {
                    yield return new TreeNode<TEntity>
                    {
                        Entity = item,
                        Children = CreateHierarchy<TEntity, TProperty>(items, item, idProperty, parentIdProperty, level, orderBy).ToList(),
                        Level = level
                    };
                }
            }
        }

        /// <summary>
        /// LINQ IEnumerable AsHierachy() extension method
        /// </summary>
        /// <typeparam name=”TEntity”>Entity class</typeparam>
        /// <typeparam name=”TProperty”>Property of entity class</typeparam>
        /// <param name=”items”>Flat collection of entities</param>
        /// <param name=”idProperty”>Reference to Id/Key of entity</param>
        /// <param name=”parentIdProperty”>Reference to parent Id/Key</param>
        /// <returns>Hierarchical structure of entities</returns>
        public static IEnumerable<TreeNode<TEntity>> AsHierarchy<TEntity, TProperty>(this IEnumerable<TEntity> items, Func<TEntity, TProperty> idProperty, Func<TEntity, TProperty> parentIdProperty, Func<TEntity, TProperty> orderBy) where TEntity : class
        {
            return CreateHierarchy(items, default(TEntity), idProperty, parentIdProperty, 0, orderBy);
        }
}
}

    public class TreeNode<T>
    {
        public TreeNode()
        {
            Children = new List<TreeNode<T>>();
        }
        public T Entity { get; set; }
        public int Level { get; set; }
        public List<TreeNode<T>> Children { get; set; }
    }