_Menu.cshtml

@{
  var ctrl = (string)ViewContext.RouteData.Values["controller"];
  var action = (string)ViewContext.RouteData.Values["action"];
  var isCurrent = new Func<string, string, string>((c, a) =>
  {
    if ((string.IsNullOrWhiteSpace(a) && c == ctrl) || (c == ctrl && a == action)) return "active";
    return "";
  });
}
<a class="nav-item nav-link @isCurrent("Root", "Index")" asp-controller="Root" asp-action="Index">Home</a>
<a class="nav-item nav-link @isCurrent("Podcast", "")" asp-controller="Podcast" asp-action="Index">Podcast</a>
<a class="nav-item nav-link @isCurrent("Root", "Calendar")" asp-controller="Root" asp-action="Calendar">Events</a>
<a class="nav-item nav-link @isCurrent("Videos", "")" asp-controller="Videos" asp-action="Index">Videos</a>
<a class="nav-item nav-link @isCurrent("Root", "About")" asp-controller="Root" asp-action="About">About</a>
<a class="nav-item nav-link @isCurrent("Root", "Contact")" asp-controller="Root" asp-action="Contact">Contact</a>
<a class="nav-item nav-link @isCurrent("Search", "")" asp-controller="Search" asp-action="Index">Search</a>