查找TREEVIEW的根节点值

private string getTreeViewRootNode(TreeView treeview)
  {
   
   TreeNode tnode = treeview.SelectedNode;
   if (tnode != null)
   {
    while(tnode.Parent != null)
    {
     tnode = tnode.Parent;
    }
    return tnode.Text;
   }
   else
   {
    return "";
   }
  }