MenuLinkTagHelper.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.Routing;
using Microsoft.AspNetCore.Mvc.TagHelpers;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Razor.TagHelpers;

namespace <%= namespace %>.TagHelpers
{
    /// <summary>
    /// <see cref="ITagHelper"/> implementation targeting &lt;menulink&gt; elements that assist with rendering contextually aware menu links.
    /// If the current route is matched the given &lt;menulink&gt; will be active. This was added to demonstrate how a TagHelper might be used
    /// with Semantic UI to implement a simple menu.
    /// </summary>
    [HtmlTargetElement("menulink", Attributes = "controller-name, action-name, menu-text")]
    public class MenuLinkTagHelper : TagHelper
    {
        public string ControllerName { get; set; }
        public string ActionName { get; set; }
        public string MenuText { get; set; }

        [ViewContext]
        public ViewContext ViewContext { get; set; }
        
        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            var urlHelper = new UrlHelper(ViewContext);

            string menuUrl = urlHelper.Action(ActionName, ControllerName);

            output.TagName = "a";
            output.Attributes.Add("href", $"{menuUrl}");
            output.Attributes.Add("class", "item blue");
            output.Content.SetContent(MenuText);

            var routeData = ViewContext.RouteData.Values;
            var currentController = routeData["controller"];
            var currentAction = routeData["action"];

            if (String.Equals(ActionName, currentAction as string, StringComparison.OrdinalIgnoreCase)
                && String.Equals(ControllerName, currentController as string, StringComparison.OrdinalIgnoreCase))
            {
                output.Attributes.SetAttribute("class", "active item blue");
            }

        }
    }
}