using System;
using System.Windows;

namespace WpfTestBinding
{
    public partial class App : Application
    {
        public static myClass _myClassTest = new myClass();

        public static myClass myClassTest
        {
            get
            {
                return _myClassTest;
            }
            set
            {
                _myClassTest = value;
            }
        }
    }
}

using System;
using System.ComponentModel;
using System.Windows;

namespace WpfTestBinding
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            this.DataContext = App.myClassTest;
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            App.myClassTest.myText = "456";
        }
    }

    public class myClass
    {
        public string myText { get; set; }
    }
}

<Window x:Class="WpfTestBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50" />
            <RowDefinition Height="50" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <TextBox Text="{Binding Path=myText, Mode=TwoWay}" Width="200" />
        <Frame Grid.Row="1" Source="PageTest.xaml" Margin="0"></Frame>
        <Button Content="Button" Grid.Row="2" Height="23" Name="button1" Width="100" Click="button1_Click" />
    </Grid>
</Window>

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.ComponentModel;

namespace WpfTestBinding
{
    public partial class PageTest : Page
    {
        public PageTest()
        {
            InitializeComponent();

            this.DataContext = App.myClassTest;

            App.myClassTest.myText = "123";
        }
    }
}

<Page x:Class="WpfTestBinding.PageTest"
      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" 
      mc:Ignorable="d" 
      d:DesignHeight="300" d:DesignWidth="300"
	Title="PageTest">

    <Grid>
        <TextBox Text="{Binding Path=myText, Mode=TwoWay}" Height="42" Name="textBox1" Width="276" />
    </Grid>
</Page>