/* Find all errors */

using System;

using System.Linq;

using System.Collections.Generic;

public static class Program

{

    public static int countIterations = 0;

    public static void Main()

    {

        try

        {

            Guid id = new Guid("418bb1ed-4edb-432d-885d-3e602a1a0246");

            List<Guid> departmentIds = new List<Guid> { id };

            List<Guid> nextIds = new List<Guid> { };

            var allDepartmentIds = GetAllDepartments(1000, id);

            var childDepartments = allDepartmentIds.Where( c => {

                countIterations++;

                return c.ParentItemId == id;

            });

            departmentIds.AddRange( childDepartments.Select( c => c.Id ) );

            nextIds.AddRange( childDepartments.Select( c => c.Id ) );

            var test = childDepartments.Select( c => c.Id ).ToList();

            childDepartments.Select( c => c.Id ).ToArray();

            Console.WriteLine(countIterations);

        }

        catch(Exception ex)

        {

            Console.WriteLine(ex.StackTrace);

        }

    }

    public class SkeletonNode

    {

        public Guid Id { get; set; }

        public Guid? ParentItemId { get; set; }

    }

    private static List<SkeletonNode> GetAllDepartments(int count, Guid lastDepartment) {

        var departments = new List<SkeletonNode>();

        Guid? parentItemId = null;

        for(int i = 0; i < count; i++)

        {

            var id = Guid.NewGuid();

            departments.Add( new SkeletonNode {

                Id = id,

                ParentItemId = parentItemId

            });

            parentItemId = id;

        }

        //В конец добавляем искомые

        departments.Add( new SkeletonNode {

                Id = lastDepartment,

                ParentItemId = null

            });

        departments.Add( new SkeletonNode {

                Id = Guid.NewGuid(),

                ParentItemId = lastDepartment

            });

        return departments;

    }

}

​