TreeViewItem firsttvi = new TreeViewItem();
                        firsttvi.Header = "Parent";
                        ToolTip tooltip = new ToolTip();
                        tooltip.Content = "Hello";
                        ToolTipService.SetToolTip(firsttvi, tooltip); 
                        TreeView1.Items.Add(firsttvi);

            TreeNode firsttvi = new TreeNode();
            firsttvi.Text = "Parent";
            firsttvi.ToolTipText = "Hello";
            treeView1.Nodes.Add(firsttvi);
            treeView1.ShowNodeToolTips = true;