How to get email address?

I am using Facebook Developer Toolkit 3.1. I am trying to use Facebook connect in my website. I do get some basic information like firstname, lastname etc. But i do not get email address which is extremely important for my site. This is my code :-

    private Api _fb;

        readonly BrowserSession _browserSession;

        private const string ApplicationKey = "myappkey";

        private const string ApplicationSecret = "myappsecret";
        private const string _emailAddress = "abc@yahoo.com";

_browserSession = new BrowserSession(ApplicationKey);
            _browserSession.SessionSecret = ApplicationSecret;

_fb = new Api(_browserSession);

            _fb.Users.GetInfoAsync(new Users.GetInfoCallback(OnGetUserComp开发者_如何学Cleted), null);

   private void OnGetUserCompleted(IList<user> users, Object state, FacebookException ex)
        {
            if (ex == null)
            {
                DispatcherHelper.Invoke(() =>
                {
                    user u = users.First();

                });
            }
        }

I have essentially stripped off the unnecessary code. I am getting user in users.First(). However there is no email address present in it. Please tell me where am i wrong.

NOTE :- I am using Silverlight for my application which is why you see the Aysnc callbacks.

Thanks in advance :)

You have to let the users of your application to give you the permission to read their email.

< fb:login-button perms="email" > Login with Facebook < /fb:login-button >

http://developers.facebook.com/docs/guides/web