// @nuget: HtmlAgilityPack

​

using System;

using HtmlAgilityPack;

public class Program

{

    public static void Main()

    {

        var withHtml = "<p>hello <b>there<1234></b></p>";

        var document = new HtmlDocument();

        document.LoadHtml(withHtml);

​

        var withoutHtml = HtmlEntity.DeEntitize(document.DocumentNode.InnerText);

        withoutHtml = HtmlEntity.DeEntitize(withoutHtml);

        Console.WriteLine(withoutHtml);

    }

}

​