private static void SetTreeViewIcon(TreeView tv, NodeType firstType, NodeType secondType, NodeType thirdType)
        {
            TreeNode tnRoot = tv.Nodes[0];
            tnRoot.ImageIndex = 0;
            foreach (TreeNode tnFirst in tnRoot.Nodes)
            {
                tnFirst.ImageIndex = tnFirst.SelectedImageIndex = (int)firstType;
                foreach (TreeNode tnSecond in tnFirst.Nodes)
                {
                    tnSecond.ImageIndex = tnSecond.SelectedImageIndex = (int)secondType;
                    foreach (TreeNode tnThird in tnSecond.Nodes)
                    {
                        tnThird.ImageIndex = tnThird.SelectedImageIndex = (int)thirdType;
                    }
                }
            }
        }