GetResponse() in C#

I have the following code in my portable class library. But it gives error that

System.Net.HttpWebRequest does not contain a definition for GetResponse().

public async Task<object> GetStateByUserId(string userID)
    {
        HttpWebRequest request;
        Stream receiveStream;
        StreamReader readStream;

        request =(HttpWebRequest)CreateGetWebRequest("state/uid/"+userID);
        using (HttpWebResponse response = (HttpWebResponse)request.GetResponse()) {

        receiveStream = response.GetResponseStream ();
        readStream = new StreamReader(receiveStream);
        string str = readStream.ReadToEnd().ToString();
        s = JsonConvert.DeserializeObject<state>(str);
            return s;
        }


    }

Anyone know why it is so?