Estou trabalhando em uma loja de aplicativos do Windows e estou tentando enviar valores de login e senha, este é o meu 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();
        }

Eu tenho muitos erros 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? )

Eu sou novo no Universal Windows Apps, então você tem alguma idéia de como posso corrigir meu código para enviar dados de login para o servidor? Obrigado pela ajuda