.net winform下TreeNode在没有子结点时也显示+号的解决办法

TreeNode竟然没有HasChildren属性，ft!

今天写程序时遇到这个问题，找了msdn及参考这里：http://support.microsoft.com/default.aspx?scid=kb%3Ben-us%3B313134，花了2个小时才搞定这个问题，特此一记。

代码如下（vs2k5上调试通过)：
using System.Runtime.InteropServices;

    class TreeNodeHelper
    {
        TreeNode treeNode = null;
        public TreeNodeHelper(TreeNode tr)
        {
            treeNode = tr;
        }

        /**//// <summary>
        /// 返回TreeNode是否有子结点
        /// </summary>
        public bool HasChildren
        {
            get
            {
                return IsTreeNodeHasChildren(treeNode);
            }
            set
            {
                MakeTreeNodeHasChildren(treeNode, value);
            }
        }

        public const UInt32 TV_FIRST = 4352;
        public const UInt32 TVSIL_NORMAL = 0;
        public const UInt32 TVSIL_STATE = 2;
        public const UInt32 TVM_SETIMAGELIST = TV_FIRST + 9;
        public const UInt32 TVM_GETNEXTITEM = TV_FIRST + 10;
        public const UInt32 TVIF_HANDLE = 16;
        public const UInt32 TVIF_STATE = 8;
        public const UInt32 TVIS_STATEIMAGEMASK = 61440;
        public const UInt32 TVM_SETITEM = TV_FIRST + 13;
        public const UInt32 TVM_GETITEM = TV_FIRST + 12;
        public const UInt32 TVGN_ROOT = 0;
        public const int TVIF_CHILDREN = 64;

        // Use a sequential structure layout to define TVITEM for the TreeView.
        [StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Auto)]
        public struct TV_ITEM
        {
            public uint mask;
            public IntPtr hItem;
            public uint state;
            public uint stateMask;
            public IntPtr pszText;
            public int cchTextMax;
            public int iImage;
            public int iSelectedImage;
            public int cChildren;
            public IntPtr lParam;
        }

        // Declare two overloaded SendMessage functions. The
        // difference is in the last parameter: one is ByVal and the
        // other is ByRef.
        [DllImport("user32.dll")]
        public static extern UInt32 SendMessage(IntPtr hWnd, UInt32 Msg,
            UInt32 wParam, UInt32 lParam);

        [DllImport("User32", CharSet = CharSet.Auto)]
        public static extern IntPtr SendMessage(IntPtr hWnd, UInt32 msg,
            UInt32 wParam, ref TV_ITEM lParam);

        /**//// <summary>
        /// 指定TreeNode是否有子结点（有子结点则TreeNode会显示+号,没有则不会显示）
        /// </summary>
        /// <param name="tr"></param>
        /// <param name="bHasChildren"></param>
        public static void MakeTreeNodeHasChildren(TreeNode tr, bool bHasChildren)
        {
            TV_ITEM tvItem = new TV_ITEM();
            tvItem.mask = TVIF_CHILDREN | TVIF_HANDLE;
            tvItem.hItem = tr.Handle;
            tvItem.cChildren = bHasChildren ? 1 : 0;
            SendMessage(tr.TreeView.Handle, TVM_SETITEM, 0, ref tvItem);
        }

        /**//// <summary>
        /// 返回树结点是否有子结点
        /// </summary>
        /// <param name="?"></param>
        /// <returns></returns>
        public static bool IsTreeNodeHasChildren(TreeNode tr)
        {
            if (tr.Nodes.Count > 0)
            {
                return true;
            }
            TV_ITEM tvItem = new TV_ITEM();
            tvItem.mask = TVIF_CHILDREN | TVIF_HANDLE;
            tvItem.hItem = tr.Handle;
            SendMessage(tr.TreeView.Handle, TVM_GETITEM, 0, ref tvItem);
            return tvItem.cChildren == 1;
        }
    }