How to make a Save/Reset options page that has two sub-pages using WPF and C#

Question

I would like to make a page that contains two sub-pages of options. The problem I've encountered is that I can't access the objects from sub-pages of the main page. I should note that I use only DataContext to script behind. And here is some of the code that will help you understand better what I mean:

StartPage.xaml

<Page x:Class="WpfApp.StartPage"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
  xmlns:local="clr-namespace:WpfApp"
  mc:Ignorable="d" 
  d:DesignHeight="300" d:DesignWidth="300"
  Title="startPage">

<Grid Background="White">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="100"/>
        <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="50"/>
    </Grid.RowDefinitions>
    <StackPanel>
        <Button Content="Page 1" Command="{Binding FirstPageCommand}"/>
        <Button Content="Page 2" Command="{Binding SecondPageCommand}"/>
    </StackPanel>
    <Frame x:Name="frame" Grid.Column="1" NavigationUIVisibility="Hidden"
           Source="Page1.xaml"/>
    <StackPanel Grid.Column="1" Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center">
        <Button Content="Reset" Margin="10" Command="{Binding ResetCommand}"/>
        <Button Content="Save" Margin="10" Command="{Binding SaveCommand}"/>
    </StackPanel>
</Grid>

StartPage.xaml.cs

using System.Windows.Controls;

namespace WpfApp
{
    /// <summary>
    /// Interaction logic for StartPage.xaml
    /// </summary>
    public partial class StartPage : Page
    {
        public StartPage()
        {
            InitializeComponent();
            DataContext = new StartPage_DataContext(this);
        }
    }
}

StartPage_DataContext.cs

using System.Windows.Controls;
using System.Windows.Input;

namespace WpfApp
{
    public class StartPage_DataContext
    {
        private Page _Page;
        public StartPage_DataContext(Page page)
        {
            _Page = page;

            FirstPageCommand = new RelayCommand(() => FirstPage());
            SecondPageCommand = new RelayCommand(() => SecondPage());
            SaveCommand = new RelayCommand(() => Save());
            ResetCommand = new RelayCommand(() => Reset());
        }

        private void FirstPage()
        {
            (_Page as StartPage).frame.NavigationService.Navigate(new Page1());
        }

        private void SecondPage()
        {
            (_Page as StartPage).frame.NavigationService.Navigate(new Page2());
        }

        private void Save()
        {
            //Here is where I need code for saving both "Page1" and "Page2" elements to Settings class.
            //Exeple : Settings._firstCB = Page1.firstCB.IsCheked.Value;
            //         Settings._secondCB = Page2.firstCB.IsCheked.Value;
        }

        private void Reset()
        {
            //Here is where I need code for setting both "Page1" and "Page2" elements to some default values.
            //Exemple : Page1.firstCB.IsCheked.Value = false;
            //          Page2.firstCB.IsCheked.Value = true;
        }

        public ICommand FirstPageCommand { get; private set; }
        public ICommand SecondPageCommand { get; private set; }
        public ICommand SaveCommand { get; private set; }
        public ICommand ResetCommand { get; private set; }
    }
}

Page1.xaml (page2 is similar the only difference is the naming of elements convention)

<Page x:Class="WpfApp.Page1"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
  xmlns:local="clr-namespace:WpfApp"
  mc:Ignorable="d" 
  d:DesignHeight="300" d:DesignWidth="300"
  Title="Page1">

<Grid Background="White">
    <StackPanel>
        <CheckBox x:Name="firstCB" Content="First Combo Box"/>
        <CheckBox x:Name="secondCB" Content="Second Combo Box"/>
        <ComboBox x:Name="firstCombo">
            <ComboBoxItem Content="First Item"/>
            <ComboBoxItem Content="Second Item"/>
        </ComboBox>
    </StackPanel>
</Grid>

Answer 1

Not sure if you have view models for your sub pages, If you do have, one way of accessing properties of those viewmodel for your check box would be as shown below.

var tt = (((_Page as StartPage).frame.NavigationService.Content as Page1).DataContext as Page1ViewModel).IsCBChecked;