SelectedItem di TabbedPage.xaml tidak berfungsi di Xamarin.Form

Hai semuanya Selamat siang Saya telah membuat tab dalam xaml, saya lebih suka xaml karena saya telah login dalam kode xaml. Kode Saya

Tab.xaml

<TabbedPage 
        xmlns="http://xamarin.com/schemas/2014/forms"
        xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
        xmlns:local="clr-namespace:FormsSample.Views;assembly=FormsSample"
        x:Class="FormsSample.Views.LoginPage"
        x:Name="TbPage">
     <TabbedPage.Children>
        <ContentPage x:Name="TbLog" Title="Login">
        </ContentPage>
        <ContentPage x:Name="TbSch" Title="Schedule">
        </ContentPage>
        <ContentPage x:Name="TbLis" Title="Customers">
        </ContentPage>
        </TabbedPage.Children>   
</TabbedPage>

Tab.xaml.cs

   namespace FormsSample.Views
   {
    public partial class LoginPage : TabbedPage 
    {

       private readonly TabbedPage _tbPage;
       private readonly ContentPage _tbList;
       private readonly ContentPage _tbLogn; 


       public LoginPage()
       {
        InitializeComponent ();
        _tbPage = this.FindByName<TabbedPage>("TbPage");
        _tbList = this.FindByName<ContentPage>("TbLis");
        _tbLogn = this.FindByName<ContentPage>("TbLog");
        RunTime();
        }

        private void RunTime()
        {
               _tbPage.CurrentPage = _tbLogn; 
           if (_tbPage.SelectedItem == _tbPage.Children[2])
           {
              DisplayAlert("Tab", "Hello World", "OK");
           }
         }

       }

     }

berubah menjadi

  _tbPage.SelectedItem == _tbList 

Hal serupa tidak terjadi apa-apa, Bagaimana cara mengatasinya? terima kasih banyak.