public class AutoSuggestTextBox: TextBox
    {
        protected override AutomationPeer OnCreateAutomationPeer()
        {
            return new AutoSuggestTextBoxAutomationPeer(this);
        }

        public ListBox SuggestionListBox;
    }

    internal class AutoSuggestTextBoxAutomationPeer : TextBoxAutomationPeer
    {
        public AutoSuggestTextBoxAutomationPeer(AutoSuggestTextBox owner) : base(owner)
        {

        }

        protected override List<AutomationPeer> GetControlledPeersCore()
        {
            List<AutomationPeer> controlledPeers = new List<AutomationPeer>();

            AutoSuggestTextBox owner = Owner as AutoSuggestTextBox;

                           controlledPeers.Add(UIElementAutomationPeer.CreatePeerForElement(owner.SuggestionListBox));

            return controlledPeers;
        }
    }