StringExtensions.cs

public static class StringExtensionMethods
{
    public static string AddHeadingAnchorsToHtml(this string html)
    {
        var doc = new HtmlDocument();
        doc.LoadHtml(html);
        // select all possible headings in the document
        var headings = doc.DocumentNode.SelectNodes("//h2");
        if (headings != null)
        {
            foreach (var heading in headings)
            {
                var headingText = heading.InnerText;
                // if heading has id, use it
                string headingId = heading.Attributes["id"]?.Value;
                if (headingId == null)
                {
                    // if heading does not have an id, generate a safe id by creating a slug based on the heading text
                    // slug is a URL/SEO friendly part of a URL, this is a good option for generating anchor fragments
                    // Source: http://predicatet.blogspot.com/2009/04/improved-c-slug-generator-or-how-to.html
                    // assumption: Prase should only contain standard a-z characters or numbers
                    headingId = headingText.ToCssClass();
                    // for the fragment to work (jump to the relevant content), the heading id and fragment needs to match
                    heading.Attributes.Append("id", headingId);
                    heading.Attributes.Append("class", "anchor");
                }

                // use a non-breaking space to make sure the heading text and the #-sign don't appear on a separate line
                //heading.InnerHtml += " ";
                // create the heading anchor which points to the heading
                //var headingAnchor = HtmlNode.CreateNode($"<a href=\"#{headingId}\" aria-label=\"Anchor for heading: {headingText}\">#</a>");
                // append the anchor behind the heading text content
                //heading.AppendChild(headingAnchor);
            }
        }

        return doc.DocumentNode.InnerHtml;
    }
    
    public static List<KeyValuePair<string, string>> GetHeadingAnchorsFromHtml(this IHtmlEncodedString html)
    {
        List<KeyValuePair<string, string>> headingData = new();

        var doc = new HtmlDocument();
        doc.LoadHtml(html.ToHtmlString());
        // select all possible headings in the document
        var headings = doc.DocumentNode.SelectNodes("//h2");
        if (headings != null)
        {
            foreach (var heading in headings)
            {
                var headingText = heading.InnerText;
                // if heading has id, use it
                string headingId = heading.Attributes["id"]?.Value;
                if (headingId == null)
                {
                    // if heading does not have an id, generate a safe id by creating a slug based on the heading text
                    // slug is a URL/SEO friendly part of a URL, this is a good option for generating anchor fragments
                    // Source: http://predicatet.blogspot.com/2009/04/improved-c-slug-generator-or-how-to.html
                    // assumption: Prase should only contain standard a-z characters or numbers
                    headingId = headingText.ToCssClass();
                    // for the fragment to work (jump to the relevant content), the heading id and fragment needs to match
                }

                headingData.Add(new KeyValuePair<string, string>(headingId, headingText));
            }
        }

        return headingData;
    }
}