Adds an "active" class to the given element when the route parameters match. An Active Route Tag Helper.

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-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 currentController = ViewContext.RouteData.Values["Controller"].ToString();
        string currentAction = ViewContext.RouteData.Values["Action"].ToString();

        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");
        }
    }
}