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_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;
        }

private void treeView1_MouseUp(object sender, MouseEventArgs e)
        {
            
            TreeNode selectnode = this.treeView1.GetNodeAt(e.Location);
            if (selectnode == null)
            {
                this.treeView1.SelectedNode = null;
                //清空内容.

            }  

        }