Live SDK SignInButton's SessionChanged event cann`t signin

Question

I use Live SDK with my WP8 app to signin skydrive,but always SignInButton's SessionChanged event handler cann`t signin.

when app start,the below code is always go else.Where is answer?            

          

private void Sesschange(object sender, Microsoft.Live.Controls.LiveConnectSessionChangedEventArgs e)
        {
         // 
            if (e.Status == LiveConnectSessionStatus.Connected)
            {
                session = e.Session;
                client = new LiveConnectClient(e.Session);
                custlist_box.Text = "Signed in.";

            }
            else
            {
                custlist_box.Text = "Not signed in.?";
                client = null;

            }
         }