how to fill textbox in a web pages using web browser?

thanks

but i use web browser,i can navigate to site but i can't change elements value like text boxes.

i use this code to fill google text box and click on search button:

       private void button1_Click(object sender, EventArgs e)
  {
         webBrowser1.Navigate("google.com"); 
         webBrowser1.DocumentCompleted += new        WebBrowserDocumentCompletedEventHandler(web_h);

  }


       private void web_h(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
           WebBrowser b = (WebBrowser)sender;
           b.Document.GetElementById("gbqfq").InnerText = "door"; 
           b.Document.GetElementById("gbqfba").InvokeMember("click");
        }

but i face with this error: System.NullReferenceException

If you are using C# or WPF you should try using WebBrowser control.
this may help : http://www.c-sharpcorner.com/UploadFile/6897bc/using-webbrowser-control/