DirectoryInfo[] subDirectories = directory.GetDirectories();
        foreach (DirectoryInfo subDir in subDirectories)
        {
           ...
        }

public static void AddNodes(FlyTreeNodeCollection nodes, string directoryPath){
    DirectoryInfo directory = new DirectoryInfo(Path.Combine(ROOT_FOLDER, directoryPath));
    if (!directory.Exists) return;
    DirectoryInfo[] subDirectories = directory.GetDirectories();
    foreach (DirectoryInfo subDir in subDirectories)
    {
        FlyTreeNode ftn = new FlyTreeNode();
        ftn.Text = HttpUtility.HtmlEncode(subDir.Name);
        ftn.Value = subDir.Name;
        ftn.NavigateUrl = subDir.FullName;
        nodes.Add(ftn);
        try
        {
            // do not set populate on demand for nodes that have no nested directories
            ftn.PopulateNodesOnDemand =  subDir.GetDirectories().Length > 0;
        }
        catch (Exception ex)
        {
            // add a child node that shows that there was an exception trying to get its child nodes
            AddExceptionNode(ftn.ChildNodes, ex);
        }
        
    }

    FileInfo[] files = directory.GetFiles();
    foreach (FileInfo file in files)
    {
        FlyTreeNode ftn = new FlyTreeNode();
        ftn.Text = HttpUtility.HtmlEncode(file.Name);
        ftn.Value = file.Name;
        ftn.NavigateUrl = file.FullName;
        nodes.Add(ftn);
    }
}