QueryableTreelist.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.Web.UI.HtmlControls.Data;

namespace MyProject.CMS.Custom.Controls
{
    public class QueryableTreelist : Sitecore.Shell.Applications.ContentEditor.TreeList 
    {
        private string _dataSource = string.Empty;
        public override string DataSource
        {
            get
            {
                if (_dataSource.StartsWith("query:"))
                {
                    if (Sitecore.Context.ContentDatabase == null || base.ItemID == null)
                        return null;
                    Item current = Sitecore.Context.ContentDatabase.GetItem(base.ItemID);

                    Item obj = null;
                    try
                    {
                        obj = Enumerable.FirstOrDefault<Item>((IEnumerable<Item>)LookupSources.GetItems(current, _dataSource));
                    }
                    catch (Exception ex)
                    {
                        Log.Error("Treelist field failed to execute query.", ex, (object)this);
                    }
                    if (obj == null)
                        return null;
                    return obj.Paths.FullPath;
                }
                return _dataSource;
            }
            set { _dataSource = value; }
        }
    }
}