问 System.Net.Http:命名空间中缺少？(使用.net 4.5)

EN

我是这个语言的新手，不知道我在做什么

到目前为止，这是我的类：

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();
                // 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....