// install first using 
// Install-Package AngleSharp
// then
using System;
using AngleSharp;
using AngleSharp.Html.Parser;

class MyClass {
    static async Task Main() {
        //Use the default configuration for AngleSharp
        var config = Configuration.Default;

        //Create a new context for evaluating webpages with the given config
        var context = BrowsingContext.New(config);

        //Source to be parsed
        var source = "<h1>Some example source</h1><p>This is a paragraph element";

        //Create a virtual request to specify the document to load (here from our fixed string)
        var document = await context.OpenAsync(req => req.Content(source));

        //Do something with document like the following
        Console.WriteLine("Serializing the (original) document:");
        Console.WriteLine(document.DocumentElement.OuterHtml);

        var p = document.CreateElement("p");
        p.TextContent = "This is another paragraph.";

        Console.WriteLine("Inserting another element in the body ...");
        document.Body.AppendChild(p);

        Console.WriteLine("Serializing the document again:");
        Console.WriteLine(document.DocumentElement.OuterHtml);
    }
}