.NET 4.x Распарсить Html-страницу - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.IO;
using System.Text.RegularExpressions;
 
namespace builder
{
 
    public partial class Form1 : Form
    {
        public Form1()
 
        {
            InitializeComponent();
 
        }
 
        public string Pars(ref string strSource, ref string strStart, ref string strEnd, [System.Runtime.InteropServices.OptionalAttribute, System.Runtime.InteropServices.DefaultParameterValueAttribute(0)] ref  
aren't supported in C#
int startPos)
        {
            int iPos;
            int iEnd;
            int lenStart = strStart.Length;
            string strResult;
            strResult = string.Empty;
            iPos = strSource.IndexOf(strStart, startPos);
            iEnd = strSource.IndexOf(strEnd, iPos + lenStart);
            if (iPos != -1 && iEnd != -1)
            {
                strResult = strSource.Substring(iPos + lenStart, iEnd - (iPos + lenStart));
            }
            return strResult;
        }
 
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
 
            WebClient WC = new WebClient();
            string proverka;
            string DarkO;
            Test = "url";
            WC.Headers.Add("Cookie: cookies);
            proverka = WC.DownloadString(Test);
 
richTextBox1.Text = proverka;
 
        }
    }
}

builedferma;test123defcom=123456789'