Включить заголовок в тело запроса

[winhttp]

нужно просто в тело запроса добавить заголовки, пытаюсь добавить UserAgent, и вылазит исключение
код

			WebProxy proxy = new WebProxy("127.0.0.1",34112);
			HttpWebRequest GET = (HttpWebRequest)HttpWebRequest.Create(@"http://site.ru/");
			GET.Proxy = proxy;
			GET.Method = "GET";
			GET.Headers.Add(HttpRequestHeader.UserAgent,"Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20100101 Firefox/17.0");
			WebResponse resp = GET.GetResponse();
			MessageBox.Show(resp.Headers.Get("Set-cookie"));

ичключение

An exception of Type System.ArgumentException was thrown:

System.ArgumentException: This header must be modified using the appropriate property or method.
   at System.Net.WebHeaderCollection.ThrowOnRestrictedHeader(String headerName)
   at System.Net.WebHeaderCollection.Add(String name, String value)
   at System.Net.WebHeaderCollection.Add(HttpRequestHeader header, String value)
   at gfgh.MainForm.Button1Click(Object sender, EventArgs e) in c:\Documents and Settings\Сашок\Мои документы\SharpDevelop Projects\gfgh\gfgh\MainForm.cs:line 40
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at gfgh.Program.Main(String[] args) in c:\Documents and Settings\...\...\SharpDevelop Projects\gfgh\gfgh\Program.cs:line 27

подскажите, как правильно добавить заголовок к запросу


Все решил. Нашел свойство GET.UserAgent

GET.UserAgent = "Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20100101 Firefox/17.0"

У меня назрел следующий вопрос - почему нельзя добавить заголовок юзерагента просто через Headers.Add ?