.Net helper to mark Navigation navbar li item as active (MVC and Pages)

Navbar Item IsSelected.md

How to set navbar item as active when user selects it, in C#

public static class HtmlHelpers
{
    public static string IsSelected(this IHtmlHelper html, string? controller = null, string? action = null, string? page = null, string? pageController = null, string? cssClass = null)
    {
        if (string.IsNullOrWhiteSpace(cssClass))
            cssClass = "active";

        // MVC controllers and actions
        string? currentAction = $"{html.ViewContext.RouteData.Values["action"]}";
        string? currentController = $"{html.ViewContext.RouteData.Values["controller"]}";
        
        // Pages
        string? currentPage = $"{html.ViewContext.RouteData.Values["page"]}";

        // Parent Menus
        if (!string.IsNullOrWhiteSpace(pageController))
        {
            var currentPageList = currentPage.Split('/').ToList();

            currentPageList.RemoveAll(i => string.IsNullOrWhiteSpace(i));

            string? currentPageController = currentPageList.Count == 2 ? currentPageList[0] : string.Empty;

            return pageController == currentPageController ? cssClass : string.Empty;
        }

        if (string.IsNullOrWhiteSpace(controller))
            controller = currentController;

        if (string.IsNullOrWhiteSpace(action))
            action = currentAction;

        if (string.IsNullOrWhiteSpace(page))
            page = currentPage;

        // return for Pages
        if (!string.IsNullOrWhiteSpace(page))
            return page == currentPage ? cssClass : string.Empty;

        // return for MVC
        return controller == currentController && action == currentAction ?
            cssClass : string.Empty;
    }
}

Used as follows:

<a class="dropdown-item @Html.IsSelected(page: "/Demo/Blank")" asp-area="" asp-page="/Demo/Blank"> Blank page </a>