Estoy trabajando en una tienda de aplicaciones de Windows, y estoy tratando de enviar valores de inicio de sesión y contraseña, este es mi código:

 try
        {
            string user = login.Text;
            string pass = password.Password;

            ASCIIEncoding encoding = new ASCIIEncoding();
            string postData = "login=" + user + "&mdp=" + pass;
            byte[] data = encoding.GetBytes(postData);

            WebRequest request = WebRequest.Create("myURL/login.php");
            request.Method = "POST";

            request.ContentType = "application/x-www-form-urlencoded";
            request.ContentLength = data.Length;

            Stream stream = request.GetRequestStream();
            stream.Write(data, 0, data.Length);
            stream.Dispose();

            WebResponse response = request.GetResponse();
            stream = response.GetResponseStream();

            StreamReader sr = new StreamReader(stream);


            sr.Dispose();
            stream.Dispose();


        }
        catch (Exception ex)
        {
            ex.Message.ToString();
        }

Tengo muchos errores como este:

'WebRequest' does not contain a definition for 'Content Length' and no extension method 'ContentLength' accepting a first argument of type 'WebRequest' was found (a using directive or an assembly reference is she missing ?)
'WebRequest' does not contain a definition for 'GetRequestStream' and no extension method 'GetRequestStream' accepting a first argument of type 'WebRequest' was found (a using directive or an assembly reference is she missing? )
'WebRequest' does not contain a definition for 'GetResponse' and no extension method 'GetResponse' accepting a first argument of type 'WebRequest' was found (a using directive or an assembly reference is she missing? )

Soy nuevo en las aplicaciones universales de Windows, así que, ¿tienen idea de cómo puedo corregir mi código para enviar datos de inicio de sesión al servidor? Gracias por la ayuda.