Asp.net treeview checkbox with Populateon demand

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ASP.NET

Hi,

How to make asp.net treeview PopulateOnDemand with checkbox. When i click the Asp.net button, treeView.CheckedNodes returns 0.

 protected void Page_Load(object sender, EventArgs e)
        {

            DirectoryInfo directory = new DirectoryInfo(@"D:\Krishna\Examples\Asp.net");
            treeSource.TreeNodePopulate += OnSourceTreeNodePopulate;
            if(Page.IsPostBack)
                return;
            treeSource.ShowCheckBoxes = TreeNodeTypes.All;
             LoadTree(directory,treeSource);
        }

        private void LoadTree(DirectoryInfo directory, TreeView treeView)
        {
            treeView.Nodes.Clear();
          
            TreeNode node = new TreeNode();
            GetFolderNode(node,directory);
            node.NavigateUrl = "javascript:void(0)";
            node.Expanded = true;
            node.Text = directory.Name;
            node.Value = directory.FullName;
            treeView.Nodes.Add(node);
            treeView.ShowLines = true;
            treeView.EnableViewState = true;
            treeView.ExpandDepth = 1;
            
        }

        public static TreeNode GetFolderNode(TreeNode node, DirectoryInfo folder)
        {
            
            TreeNode folderNode = null;
            DirectoryInfo[] folders = folder.GetDirectories();

            DirectoryInfo[] subDirectories = folder.GetDirectories();

            // Scan the directories in the current directory and call this method 
            // again to go one level into the directory tree
            foreach (DirectoryInfo subDirectory in subDirectories)
            {
                folderNode = new TreeNode();
                folderNode.NavigateUrl = string.Empty;
                folderNode.Text = subDirectory.Name;
                folderNode.ShowCheckBox = true;
                folderNode.PopulateOnDemand = true;
                //folderNode.ChildNodes.Add(GetFolderNode(folderNode,subDirectory));
                node.ChildNodes.Add(folderNode);
                
            }
                return node;

            
        }


        private void OnSourceTreeNodePopulate(object sender, TreeNodeEventArgs e)
        {
            if (!Page.IsCallback)
                return;

            DirectoryInfo directory = new DirectoryInfo(e.Node.ValuePath);
            GetFolderNode(e.Node, directory);
        }

        protected void OnCopyClicked(object sender, EventArgs e)
        {
            if(treeSource.CheckedNodes.Count==0)
                return;
       }
}

Dont want to postback for each check, Loading all folders makes page very slower.

Thanks and regards
Krishna