public interface IAccountContract
    {
        string Login(string account, string password);
    }

    [Export(typeof(IAccountContract))]
    public class AccountRealize : IAccountContract
    {
        public string Message;

        public AccountRealize(string message)
        {
            Message = message;
        }

        public string Login(string account, string password)
        {
            return string.Format("账户{0}通过密码{1}登陆成功！附加信息：{2}。", account, password, Message);
        }
    }

    [Export]
    public class MEFController : Controller
    {
        [Import]
        public IAccountContract AccountRealize { get; set; }

        //
        // GET: /MEF/

        public ActionResult Index()
        {
            string result = AccountRealize.Login("AAA", "123123");

            return Content(result);
        }

    }