System.Net.Http: missing from namespace?

Gere is my class so far:

using System;using System.Collections.Generic;using System.Net.Http;using System.Web;using System.Net;using System.IO;public class MyClass    {        private const string URL = "https://sub.domain.com/objects.json?api_key=123";        private const string data = @"{""object"":{""name"":""Title""}}";        public static void CreateObject()        {            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URL);            request.Method = "POST";            request.ContentType = "application/json";            request.ContentLength = data.Length;            StreamWriter requestWriter = new StreamWriter(request.GetRequestStream(), System.Text.Encoding.ASCII);            requestWriter.Write(data);            requestWriter.Close();            try            {                // get the response                WebResponse webResponse = request.GetResponse();                Stream webStream = webResponse.GetResponseStream();                StreamReader responseReader = new StreamReader(webStream);                string response = responseReader.ReadToEnd();                responseReader.Close();            }            catch (WebException we)            {                string webExceptionMessage = we.Message;            }            catch (Exception ex)            {                // no need to do anything special here....            }        }        static void Main(string[] args)        {            MyClass.CreateObject();        }}

when I do csc filename.cs, I get the following error:

The type or namespace name 'Http' does not exist in the namespace 'System.Net' (are you missing an assembly reference?)

I'm using .NET 4.5