C#

exe -> 'a' -> 'c' -> 'd'
    -> 'b' ----^

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace Bouwgegevens.Gebouwen
{
    public partial class Plan : System.Web.UI.Page
    {

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!ScriptManager.GetCurrent(this).IsInAsyncPostBack)
            {
                PopulateMenu();
            }
            else
            {
                //code when it is an async postback
            }
        }
        private void PopulateMenu()
        {
            List<tbl_NEN2634> allMenu = new List<tbl_NEN2634>();

            using (DBBouwgegevensEntities dc = new DBBouwgegevensEntities())
            {
                allMenu = dc.tbl_NEN2634.ToList();
            }
            // Call function here for bind treeview
            CreateTreeView(allMenu, 0, null);
        }

        public List<tbl_Objecten_tbl_NEN2634> TheList
        {
            get;set;
        }

        private void CreateTreeView(List<tbl_NEN2634> source, int parentID, TreeNode parentNode)
        {

            int objectID = Convert.ToInt32(Request.QueryString["ObjectID"]);

            List<tbl_NEN2634> newSource = (from b in source
                                           join c in TheList
                                           on b.NEN2634ID equals c.NEN2634ID
                                           where b.ParentID == parentID
                                           select b).ToList();

            //List<tbl_NEN2634> newSource = source.Where(a => a.ParentID.Equals(parentID)).ToList();

            foreach (var i in newSource)
            {
                TreeNode newnode = new TreeNode("" + i.NEN2634_Code + "" + " " + i.NEN2634_Titel, i.NEN2634ID.ToString());
                if (parentNode == null)
                {
                    trvPlan.Nodes.Add(newnode);
                }
                else
                {
                    parentNode.ChildNodes.Add(newnode);
                }
                CreateTreeView(source, i.NEN2634ID, newnode);
            }
        }
    }
}