Почему не работает проверка на TabIndex?

    <TextBox TabIndex="0" Style="{StaticResource TextBox}"   Margin="0 10 0 0" Width="230"  Name="tBLogin"/>
  <PasswordBox   TabIndex="1" PasswordChanged="pBPassword_PasswordChanged"  Name="pBPassword" Width="230"/>

                    <TextBox TabIndex="2" TextChanged="tBPassword_TextChanged"
                             x:Name="tBPassword" 
                             Style="{StaticResource TextBox}" 
                             Width="230" Visibility="Collapsed"/>

Вот проверка

 private void window_Loaded(object sender, RoutedEventArgs e)
        {                            
            if (TabIndex == 0)
            {
                iconLogin.Foreground = Brushes.Blue;
                iconPassword.Foreground = Brushes.Gray;
            }
            else if (TabIndex == 1 || TabIndex == 2)
            {                
                iconPassword.Foreground = Brushes.Blue;
                iconLogin.Foreground = Brushes.Gray;
            }            
        }

        private void window_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            if (TabIndex == 0)
            {
                iconLogin.Foreground = Brushes.Blue;
                iconPassword.Foreground = Brushes.Gray;
            }
            else if (TabIndex == 1 || TabIndex == 2)
            {
                iconPassword.Foreground = Brushes.Blue;
                iconLogin.Foreground = Brushes.Gray;
            }
        }

Если поменять в коде TabIndex на Focus() == true или IsFocused == true То так тоже не работает