public class AutoCompleteLookupFieldControl : BaseFieldControl
    {
        protected override void CreateChildControls()
        {
            base.CreateChildControls();            
            if (!FindScriptManager(Page.Controls))
            {
                Controls.Add(new ScriptManager());
            }
        }

        private bool FindScriptManager(ControlCollection controls)
        {
            foreach (Control item in controls)
            {
                if (item is ScriptManager)
                {
                    return true;
                }
                if (FindScriptManager(item.Controls))
                {
                    return true;
                }
            }
            return false;
        }
    }