HttpClient does not exist in .net 4.0: what can I do?

Ok i edited my code i dont get errors but the messageBox.Show return nothing empty box. Maybe i need to add something in the referrer string ? I didnt understand what is the referrer and what should i put there. And i have a key already im using it in my code. The key is a long string and im using it in my code i dont use with the referrer. Why it dosent translate the word "hi" ?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Globalization;
using System.IO;
using System.Net;
using System.Web;
using System.Web.Script.Serialization;




namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        private JavaScriptSerializer _Serializer = new JavaScriptSerializer();

        public Form1()
        {
            InitializeComponent();
            string f = TranslateText("hi", "English", "German", "", "");
            MessageBox.Show(f);
        }

        private void Form1_Load(object sender, EventArgs e)
        {
        }

        public string TranslateText(string inputText, string sourceLanguage, string destinationLanguage, string referrer, string apiKey)
        {
                string requestUrl = string.Format(
                    "http://ajax.googleapis.com/ajax/services/language/translate?v=1.0&q={0}&langpair={1}|{2}&key={3}", 
                    HttpUtility.UrlEncode(inputText), 
                    sourceLanguage.ToLowerInvariant(), 
                    destinationLanguage.ToLowerInvariant(), 
                    apiKey
                );

                try
                {
                    HttpWebRequest http = (HttpWebRequest)HttpWebRequest.Create(requestUrl);
                    http.Referer = referrer;
                    HttpWebResponse response = (HttpWebResponse)http.GetResponse();
                    using (StreamReader sr = new StreamReader(response.GetResponseStream()))
                    {
                        string responseJson = sr.ReadToEnd();
                        var translation = this._Serializer.Deserialize<Milkshake.Integration.Google.GoogleAjaxResponse<Milkshake.Integration.Google.Translate.TranslationResponse>>(responseJson);

                        if (translation != null && translation.ResponseData != null && translation.ResponseData.ResponseStatus == HttpStatusCode.OK)
                        {
                            return translation.ResponseData.TranslatedText;
                        }
                        else
                        {
                            return String.Empty;
                        }
                    }
                }
            catch
                {
                    return String.Empty;
            }
        }
    }
}

I've used HttpClient in .NET 4.0 applications on numerous occasions. If you are familiar with NuGet, you can do an Install-Package Microsoft.Net.Http to add it to your project. See the link below for further details.

http://nuget.org/packages/Microsoft.Net.Http

• You can use WebClient.

• Or (if you need more fine-grained control over the request) HttpWebRequest

• Or, HttpClient in System.Net.Http.dll.

Here's a "translation" to HttpWebRequest (needed rather than WebClient in order to set the referrer). (Uses System.Net and System.IO):

    HttpWebRequest http = (HttpWebRequest)HttpWebRequest.Create(requestUrl))
    http.Referer = referrer;
    HttpWebResponse response = (HttpWebResponse )http.GetResponse();
    using (StreamReader sr = new StreamReader(response.GetResponseStream()))
    {
        string responseJson = sr.ReadToEnd();
        // more stuff
    }

Referring to the answers above, I am only adding this to help clarify things. It is possible to use HttpClient from .Net 4.0, and you have to install the package from here

However, the text is very confusion and contradicts itself.

This package is not supported in Visual Studio 2010, and is only required for projects targeting .NET Framework 4.5, Windows 8, or Windows Phone 8.1 when consuming a library that uses this package.

But underneath it states that these are the supported platforms.

Supported Platforms:

• .NET Framework 4

• Windows 8

• Windows Phone 8.1

• Windows Phone Silverlight 7.5

• Silverlight 4

• Portable Class Libraries

Ignore what it ways about targeting .Net 4.5. This is wrong. The package is all about using HttpClient in .Net 4.0. However, you may need to use VS2012 or higher. Not sure if it works in VS2010, but that may be worth testing.

read this...

Portable HttpClient for .NET Framework and Windows Phone

see paragraph Using HttpClient on .NET Framework 4.0 or Windows Phone 7.5 http://blogs.msdn.com/b/bclteam/archive/2013/02/18/portable-httpclient-for-net-framework-and-windows-phone.aspx

Agreeing with TrueWill's comment on a separate answer, the best way I've seen to use system.web.http on a .NET 4 targeted project under current Visual Studio is Install-Package Microsoft.AspNet.WebApi.Client -Version 4.0.30506