From a ESRI.ArcGIS.Desktop.AddIns.Button, I would like to get the list box from the dockable window; see below.  Where in the dockable window code can I set the User Data property?  Is this the correct approach?
        protected override void OnClick()
        {
            IDockableWindowManager dockWindowManager = ArcMap.DockableWindowManager as IDockableWindowManager;
            if (dockWindowManager != null)
            {
                UID windowID = new UIDClass();
                windowID.Value = "DocWin";
                IDockableWindow docWin = dockWindowManager.GetDockableWindow(windowID);
                if (docWin.IsVisible())
                {
                    docWin.Show(false);
                }
                else 
                {
                    docWin.Show(true);
                }
                System.Windows.Forms.ListBox containedBox = docWin.UserData as System.Windows.Forms.ListBox;
                if (containedBox != null) containedBox.Items.Add("Test 1");
            }
        }
---------------
Docable window class code
 /// <summary>
    /// Designer class of the dockable window add-in. It contains user interfaces that
    /// make up the dockable window.
    /// </summary>
    public partial class DocWin : UserControl
    {
        public DocWin(object hook)
        {
            InitializeComponent();
            this.Hook = hook;
        }
        /// <summary>
        /// Host object of the dockable window
        /// </summary>
        private object Hook
        {
            get;
            set;
        }
        /// <summary>
        /// Implementation class of the dockable window add-in. It is responsible for 
        /// creating and disposing the user interface class of the dockable window.
        /// </summary>
        public class AddinImpl : ESRI.ArcGIS.Desktop.AddIns.DockableWindow
        {
            private DocWin m_windowUI;
            public AddinImpl()
            {
            }
            protected override IntPtr OnCreateChild()
            {
                m_windowUI = new DocWin(this.Hook);
                return m_windowUI.Handle;
            }
            protected override void Dispose(bool disposing)
            {
                if (m_windowUI != null)
                    m_windowUI.Dispose(disposing);
                base.Dispose(disposing);
            }
        }
    }