using System;

using HtmlAgilityPack;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string html = @"<h1> Title 1 </h1>

<p>Lorem ipsum dolor...</p>

<h2> Title 2 </h2>

<h2> Title 2 Different </h2>";

​

        string resultHtml = ReplaceHeaderHtml(html);

        Console.Write(resultHtml);

    }

    private static string ReplaceHeaderHtml(string html)

{

    var doc = new HtmlDocument();

    doc.LoadHtml(html);

    var xpath = "//*[self::h1 or self::h2 or self::h3 or self::h4]";

    HtmlNodeCollection headers = doc.DocumentNode.SelectNodes(xpath);

    if (headers == null || headers.Count == 0)

        return html;

​

    var headerList = headers

        .Where(node => !"a".Equals(node.PreviousSibling?.OriginalName, StringComparison.InvariantCultureIgnoreCase))

        .ToList();

​

    if (!headerList.Any())

        return html;

​

    for(int i = 0; i < headerList.Count; i++)

    {

        var header = headerList[i];

        var parentNode = header.ParentNode;

        int headerIndex = parentNode.ChildNodes.IndexOf(header);

        HtmlNode div = doc.CreateElement("div");

        HtmlNode anchor = doc.CreateElement("a");

​

        string href;

        switch(header.OriginalName)

        {

            case "h1": href = "#my_slugged__link_by_title"; break;

            case "h2": href = "#my_slugged__link_by_title_2"; break;

            case "h3": href = "#my_slugged__link_by_title_3"; break;

            default: href = "#my_slugged__link_by_title"; break;

        }

​

        anchor.Attributes.Add("class", "header_link");

        anchor.Attributes.Add("href", href);

​

        div.ChildNodes.Add(anchor);

        div.ChildNodes.Add(header);

        parentNode.ChildNodes.Remove(header);

        parentNode.ChildNodes.Insert(headerIndex, div);

    }

​

    return doc.DocumentNode.OuterHtml;

}

}