錯誤CS0246：找不到類型或命名空間名稱“HtmlAgilityPack”

我目前正在開發一個自然語言處理程序，我在其中訪問Google Translate作為我的字典，以便翻譯用戶輸入。

using UnityEngine;
using System.Collections;
using System.Text;
using System.Net;
public class GoogleTranslate : MonoBehaviour {
    public static string Translate(string input, string languagePair, Encoding encoding)
    {
        string url = String.Format("http://www.google.com/translate_t?hl=en&ie=UTF8&text=       {0}&langpair={1}", input, languagePair);
        string result = String.Empty;

        using (WebClient webClient = new WebClient())
        {
            webClient.E ncoding = encoding;
            result = webClient.DownloadString(url);
        }

        HtmlDocument doc = new HtmlDocument();
        doc.LoadHtml(result);
        return doc.DocumentNode.SelectSingleNode("//textarea[@name='utrans']").InnerText;
    }
}

當我在Assembly中編譯這個程序時，我實際上使用的是Unity，但它使用Assembly編譯，我得到了返回錯誤：

using UnityEngine;
using System.Collections;
using System.Text;
using System.Net;
public class GoogleTranslate : MonoBehaviour {
    public static string Translate(string input, string languagePair, Encoding encoding)
    {
        string url = String.Format("http://www.google.com/translate_t?hl=en&ie=UTF8&text=       {0}&langpair={1}", input, languagePair);
        string result = String.Empty;

        using (WebClient webClient = new WebClient())
        {
            webClient.E ncoding = encoding;
            result = webClient.DownloadString(url);
        }

        HtmlDocument doc = new HtmlDocument();
        doc.LoadHtml(result);
        return doc.DocumentNode.SelectSingleNode("//textarea[@name='utrans']").InnerText;
    }
}

我開始在網上查找HtmlDocument的正確命名空間，並閱讀我應該寫的：

using UnityEngine;
using System.Collections;
using System.Text;
using System.Net;
public class GoogleTranslate : MonoBehaviour {
    public static string Translate(string input, string languagePair, Encoding encoding)
    {
        string url = String.Format("http://www.google.com/translate_t?hl=en&ie=UTF8&text=       {0}&langpair={1}", input, languagePair);
        string result = String.Empty;

        using (WebClient webClient = new WebClient())
        {
            webClient.E ncoding = encoding;
            result = webClient.DownloadString(url);
        }

        HtmlDocument doc = new HtmlDocument();
        doc.LoadHtml(result);
        return doc.DocumentNode.SelectSingleNode("//textarea[@name='utrans']").InnerText;
    }
}

把它放到我的程序後，我收到了錯誤：

using UnityEngine;
using System.Collections;
using System.Text;
using System.Net;
public class GoogleTranslate : MonoBehaviour {
    public static string Translate(string input, string languagePair, Encoding encoding)
    {
        string url = String.Format("http://www.google.com/translate_t?hl=en&ie=UTF8&text=       {0}&langpair={1}", input, languagePair);
        string result = String.Empty;

        using (WebClient webClient = new WebClient())
        {
            webClient.E ncoding = encoding;
            result = webClient.DownloadString(url);
        }

        HtmlDocument doc = new HtmlDocument();
        doc.LoadHtml(result);
        return doc.DocumentNode.SelectSingleNode("//textarea[@name='utrans']").InnerText;
    }
}

我在網上看到我必須更具體和使用：

using UnityEngine;
using System.Collections;
using System.Text;
using System.Net;
public class GoogleTranslate : MonoBehaviour {
    public static string Translate(string input, string languagePair, Encoding encoding)
    {
        string url = String.Format("http://www.google.com/translate_t?hl=en&ie=UTF8&text=       {0}&langpair={1}", input, languagePair);
        string result = String.Empty;

        using (WebClient webClient = new WebClient())
        {
            webClient.E ncoding = encoding;
            result = webClient.DownloadString(url);
        }

        HtmlDocument doc = new HtmlDocument();
        doc.LoadHtml(result);
        return doc.DocumentNode.SelectSingleNode("//textarea[@name='utrans']").InnerText;
    }
}

現在我把它放進去了，我仍然繼續收到錯誤：

using UnityEngine;
using System.Collections;
using System.Text;
using System.Net;
public class GoogleTranslate : MonoBehaviour {
    public static string Translate(string input, string languagePair, Encoding encoding)
    {
        string url = String.Format("http://www.google.com/translate_t?hl=en&ie=UTF8&text=       {0}&langpair={1}", input, languagePair);
        string result = String.Empty;

        using (WebClient webClient = new WebClient())
        {
            webClient.E ncoding = encoding;
            result = webClient.DownloadString(url);
        }

        HtmlDocument doc = new HtmlDocument();
        doc.LoadHtml(result);
        return doc.DocumentNode.SelectSingleNode("//textarea[@name='utrans']").InnerText;
    }
}

我想知道用於HtmlDocument的命名空間。任何有關此事的幫助將不勝感激！