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Linking and updating dependency properties in user control

I created a simplified version of my code that is experiencing the same problem. The problem is that I'm not sure why the dependency property in my custom control is not updated when it changes in the model.

Model:

public class MainWindowModel : INotifyPropertyChanged
{
    private bool isChecked;
    public bool IsChecked { get { return isChecked; } set { isChecked = value; OnPropertyChanged("IsChecked"); } }

    public event PropertyChangedEventHandler PropertyChanged;
    void OnPropertyChanged(string prop)
    {
        if (this.PropertyChanged != null)
            this.PropertyChanged(this, new PropertyChangedEventArgs(prop));
    }
}

XAML:

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:custom="clr-namespace:WpfApplication1"
    Title="MainWindow" Height="350" Width="525">
<Grid>
    <custom:CustomTextbox x:Name="TextboxName" HorizontalAlignment="Center" VerticalAlignment="Center" Width="200" TextChanged="CustomTextbox_TextChanged">
        <custom:CustomTextbox.CustomTextboxItems>
            <custom:CustomTextboxItem IsChecked="{Binding IsChecked}" />
        </custom:CustomTextbox.CustomTextboxItems>
    </custom:CustomTextbox>

    <Button Content="Do It" Click="Button_Click" HorizontalAlignment="Center" VerticalAlignment="Bottom" Margin="0,0,0,20" />
</Grid>
</Window>

Code for:

public partial class MainWindow : Window
{
    MainWindowModel model;

    public MainWindow()
    {
        InitializeComponent();

        model = new MainWindowModel();
        this.DataContext = model;
    }

    private void CustomTextbox_TextChanged(object sender, TextChangedEventArgs e)
    {
        model.IsChecked = true;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (TextboxName.CustomTextboxItems[0].IsChecked)
        {
            TextboxName.Text = "Property successfully changed";
        }
    }
}

User control:

public class CustomTextbox : TextBox
{
    public CustomTextbox()
    {
        CustomTextboxItems = new ObservableCollection<CustomTextboxItem>();
    }

    public ObservableCollection<CustomTextboxItem> CustomTextboxItems { get; set; }
}

public class CustomTextboxItem : FrameworkElement
{
    public static readonly DependencyProperty IsCheckedProperty = DependencyProperty.Register("IsChecked", typeof(bool), typeof(CustomTextboxItem), new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

    public bool IsChecked
    {
        get { return (bool)GetValue(IsCheckedProperty); }

        set { SetValue(IsCheckedProperty, value); }
    }
}

As you can see in the user control, I have a set of elements containing objects with dependent properties with which I want to bind. Therefore, I create objects in xaml and adjust the binding, but when I update the binded property in the model, it does not change it in the user control. Any ideas?

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Visual Studio. , -, , .

CustomTextBox CustomTextBoxItem, . . , , .

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