EMail'在未引用的程序集中定义

我得到以下错误:

Error   1   The type 'Google.GData.Extensions.EMail' is defined in an assembly that is not referenced. You must add a reference to assembly 'Google.GData.Extensions, Version=1.9.0.0, Culture=neutral, PublicKeyToken=0b4c5df2ebf20876'

using Google.Contacts;
using Google.GData;
using Google.GData.Contacts;
using Google.GData.Client;
using Google.GData.Extensions;
   class Program
    {
        static void Main(string[] args)
        {
            RequestSettings settings = new RequestSettings("test app", "mail", "pass");
            settings.AutoPaging = true;
            ContactsRequest contacts = new ContactsRequest(settings);
            Feed<Contact> list = contacts.GetContacts();
            foreach (Contact contact in list.Entries)
            {
                Console.WriteLine(contact.Emails);
            }
            Console.ReadLine();

        }
    }

我如何修复它?