解决treeview未选中时，默认选中首个根节点的问题！

 private void treeView1_MouseUp(object sender, MouseEventArgs e)
         {

             TreeNode selectnode = this.treeView1.GetNodeAt(e.Location);
             if (selectnode == null)
             {
                 this.treeView1.SelectedNode = null;

             }  

         }

         private void treeView1_BeforeSelect(object sender, TreeViewCancelEventArgs e)
         {
             if (e.Action == TreeViewAction.Unknown)
                 e.Cancel = true;
         }