OnBeforeExpand with Dictionary

protected async override void OnBeforeExpand(TreeViewCancelEventArgs e)
{

    if (!_expandedCache.Contains(e.Node.FullPath))
    {
        TreeNodeCollection tnc = e.Node.Nodes;
        _expandedCache.Add(e.Node.FullPath);
        await Task.Run(() => {
            Dictionary<ulong, ShellFileGetInfo.FolderIcons> icons = new Dictionary<ulong, ShellFileGetInfo.FolderIcons>();
            ShellFileGetInfo.FolderIcons fi;
            string curPath;
            foreach (TreeNode n in tnc)
            {
                curPath = Path.Combine((string)Tag, n.FullPath.Replace('/', Path.DirectorySeparatorChar));
                if (File.Exists(Path.Combine(curPath, "desktop.ini")) == true)
                {
                    if (File.ReadAllText(Path.Combine(curPath, "desktop.ini")).Contains("IconFile"))
                    {
                        fi = ShellFileGetInfo.GetFolderIcon(curPath, false);
                        if (fi.closed != null || fi.open != null)
                        {
                            icons.Add(((NtfsUsnJournal.UsnEntry)n.Tag).FileReferenceNumber, fi);
                        }
                    }
                }
                //EndUpdate();
            }

            if (icons.Count > 0)
            {
                Invoke((MethodInvoker)(() =>
                {

                    List<Image> images = new List<Image>();
                    foreach (var icon in icons)
                    {
                        images.Add(icon.Value.closed.ToBitmap());
                        images.Add(icon.Value.open.ToBitmap());
                    }

                    BeginUpdate();

                    int imageIndexCount = ImageList.Images.Count;

                    ImageList.Images.AddRange(images.ToArray());
                    foreach (var icon in icons)
                    {
                        e.Node.Nodes[icon.Key.ToString()].ImageIndex = imageIndexCount;
                        imageIndexCount++;
                        e.Node.Nodes[icon.Key.ToString()].SelectedImageIndex = imageIndexCount;
                        imageIndexCount++;
                    }

                    EndUpdate();
                }));
            }
        });

    }
    base.OnBeforeExpand(e);
}