li helper to look at the requested route and add a 'selected' css class to the li tag

ItemBuild.cshtml

@helper ItemBuild(string menuItemText, string controllerName, string actionName){
    var controller = ViewContext.Controller.ValueProvider.GetValue("controller").RawValue;
    var action = ViewContext.Controller.ValueProvider.GetValue("action").RawValue;
    <li @(controller.Equals(controllerName) && action.Equals(actionName) ? "class=selected" : string.Empty)>@Html.ActionLink(menuItemText, actionName, controllerName)</li>}