Code
private void treeView1_DragDrop(object sender, DragEventArgs e)
        {
            //判断数据是否是 TreeNode 的格式
            if (e.Data.GetDataPresent("System.Windows.Forms.TreeNode", false))
            {
                TreeNode dragedTreeNode = (TreeNode)e.Data.GetData("System.Windows.Forms.TreeNode");

                //获取鼠标落点处 TreeNode
                Point pt = ((TreeView)sender).PointToClient(new Point(e.X, e.Y));
                TreeNode targetTreeNode = ((TreeView)sender).GetNodeAt(pt);

                targetTreeNode.Nodes.Add(dragedTreeNode.Clone() as TreeNode);
                dragedTreeNode.Remove();

                (sender as TreeView).ExpandAll();
            }
            else
            {
                MessageBox.Show("不是有效的树节点！");
            }
        }