To add Active class on nav link in asp.net core

ActiveRouteTagHelper.cs

[HtmlTargetElement(Attributes = "is-active-route")]
    public class ActiveRouteTagHelper : TagHelper
    {
        private IDictionary<string, string> _routeValues;

        /// <summary>The name of the action method.</summary>
        /// <remarks>Must be <c>null</c> if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" /> is non-<c>null</c>.</remarks>
        [HtmlAttributeName("asp-page")]
        public string Page { get; set; }

        /// <summary>The name of the action method.</summary>
        /// <remarks>Must be <c>null</c> if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" /> is non-<c>null</c>.</remarks>
        [HtmlAttributeName("asp-action")]
        public string Action { get; set; }

        /// <summary>The name of the controller.</summary>
        /// <remarks>Must be <c>null</c> if <see cref="P:Microsoft.AspNetCore.Mvc.TagHelpers.AnchorTagHelper.Route" /> is non-<c>null</c>.</remarks>
        [HtmlAttributeName("asp-controller")]
        public string Controller { get; set; }

        /// <summary>Additional parameters for the route.</summary>
        [HtmlAttributeName("asp-all-route-data", DictionaryAttributePrefix = "asp-route-")]
        public IDictionary<string, string> RouteValues
        {
            get
            {
                if (this._routeValues == null)
                    this._routeValues =
                        (IDictionary<string, string>) new Dictionary<string, string>(
                            (IEqualityComparer<string>) StringComparer.OrdinalIgnoreCase);
                return this._routeValues;
            }
            set { this._routeValues = value; }
        }

        /// <summary>
        /// Gets or sets the <see cref="T:Microsoft.AspNetCore.Mvc.Rendering.ViewContext" /> for the current request.
        /// </summary>
        [HtmlAttributeNotBound]
        [ViewContext]
        public ViewContext ViewContext { get; set; }

        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            base.Process(context, output);

            if (ShouldBeActive())
            {
                MakeActive(output);
            }

            output.Attributes.RemoveAll("is-active-route");
        }

        private bool ShouldBeActive()
        {
            string currentPage = String.Empty;
            string currentController = String.Empty;
            string currentAction = String.Empty;
            if (ViewContext.RouteData.Values["Page"] != null)
            {
                currentPage = ViewContext.RouteData.Values["Page"].ToString();
            }
            else
            {
                currentController = ViewContext.RouteData.Values["Controller"].ToString();
                currentAction = ViewContext.RouteData.Values["Action"].ToString();
            }

            if (!string.IsNullOrWhiteSpace(Page) && Page.ToLower() != currentPage.ToLower())
            {
                return false;
            }

            if (!string.IsNullOrWhiteSpace(Controller) && Controller.ToLower() != currentController.ToLower())
            {
                return false;
            }

            if (!string.IsNullOrWhiteSpace(Action) && Action.ToLower() != currentAction.ToLower())
            {
                return false;
            }

            foreach (KeyValuePair<string, string> routeValue in RouteValues)
            {
                if (!ViewContext.RouteData.Values.ContainsKey(routeValue.Key) ||
                    ViewContext.RouteData.Values[routeValue.Key].ToString() != routeValue.Value)
                {
                    return false;
                }
            }

            return true;
        }

        private void MakeActive(TagHelperOutput output)
        {
            var classAttr = output.Attributes.FirstOrDefault(a => a.Name == "class");
            if (classAttr == null)
            {
                classAttr = new TagHelperAttribute("class", "active");
                output.Attributes.Add(classAttr);
            }
            else if (classAttr.Value == null || classAttr.Value.ToString().IndexOf("active") < 0)
            {
                output.Attributes.SetAttribute("class", classAttr.Value == null
                    ? "active"
                    : classAttr.Value.ToString() + " active");
            }
        }
    }

use as
<ul class="navbar-nav"> <li class="nav-item" is-active-route asp-action="Index" asp-controller="Search"> <a class="nav-link" asp-action="Index" asp-controller="Search">🔎 Search</a> </li> </ul>