props/Props/TagHelpers/NavLinkTagHelper.cs

using System;
using System.Linq;
using System.Text.Encodings.Web;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
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 Props.TagHelpers
{
    public class NavLinkTagHelper : AnchorTagHelper
    {
        private IUrlHelperFactory urlHelperFactory;
        private IUrlHelper urlHelper;
        public NavLinkTagHelper(IHtmlGenerator generator, IUrlHelperFactory urlHelperFactory) : base(generator)
        {
            this.urlHelperFactory = urlHelperFactory;
        }

        public override void Init(TagHelperContext context)
        {
            this.urlHelper = urlHelperFactory.GetUrlHelper(ViewContext);
            base.Init(context);
        }

        public string ActiveClass { get; set; } = "active";

        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            base.Process(context, output);
            TagHelperAttribute dest;
            if (output.Attributes.TryGetAttribute("href", out dest) && urlHelper.RouteUrl(ViewContext.RouteData.Values).Equals(dest.Value))
            {
                output.AddClass(ActiveClass, HtmlEncoder.Default);
                output.Attributes.Add("aria-current", "page");
            }
            output.TagName = "a";
        }
    }
}