Fetching a Web page source code and retriving its controls.

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C#

ASP.NET

Hello Programmers,
this is my first question at code project site and I wish I could get an answer - my best programmer friend suggested this site to me as the best - and I'm searching for a solution for;
I need to get selected data from an external web page...
I've tried by fetching the desired page source code and I could do that by the following code:

...
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.IO;

...
        private void BtnGetSc_Click(object sender, EventArgs e)
        {
            string url = TbUrl.Text;

            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
            HttpWebResponse res = (HttpWebResponse)req.GetResponse();

            StreamReader sr = new StreamReader(res.GetResponseStream());

            RtbSC.Text = sr.ReadToEnd();
            sr.Close();
        }
...

but I'm stuck in converting this source code to a C# controls, so any suggestion??!

Hint: in outher words; I need to read any web page and fetch a text box value from it.
thanks in advance for any reply :)