Adds an "active" class to the given element when the route parameters match. An Active Page Tag Helper for use with Razor Pages.

ActivePageTagHelper.cs

[HtmlTargetElement(Attributes = "is-active-page")]
public class ActivePageTagHelper : TagHelper
{
    /// <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>
    /// 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-page");
    }

    private bool ShouldBeActive()
    {
        string currentPage = ViewContext.RouteData.Values["Page"].ToString();

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