Fast way to parse and edit HTML from a website

See more:

C#

HTML

stream

web

parsing

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net;
using System.IO;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private String webText;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            WebClient web = new WebClient();
            System.IO.Stream stream = web.OpenRead("https://de.wikipedia.org");
            using (System.IO.StreamReader reader = new System.IO.StreamReader(stream))
            {
                webText = reader.ReadToEnd();   
            }
            stream.Close();

            richTextBox1.Text = webText;
            }   
    }
}

What I have tried:

This Code is working quite well to display the whole sourcecode.

But i would like to go through the sourcecode using the getElementById function.
Apparently this function is limited to htmlDocument types and i couldnt find a way to convert the string i get back from my stream into a htmlDocument.

Is there a way to convert into htmlDocument from string?
Or instead of writing the sourcecode into a string, can i create a htmlDocument in the first place?

Thanks