Mvvm - model class - what is 'best' way of accessing private members and properties from within the class?

Quote:

Model–View–ViewModel (MVVM) is a software architectural pattern.

MVVM facilitates a separation of development of the graphical user interface – be it via a markup language or GUI code – from development of the business logic or back-end logic (the data model). The view model of MVVM is a value converter,[1] meaning the view model is responsible for exposing (converting) the data objects from the model in such a way that objects are easily managed and presented. In this respect, the view model is more model than view, and handles most if not all of the view's display logic.[1] The view model may implement a mediator pattern, organizing access to the back-end logic around the set of use cases supported by the view.

<Window x:Class="WpfSimpleBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        mc:Ignorable="d"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

        xmlns:local="clr-namespace:WpfSimpleBinding"

        Title="Basic code-behin binding example"
        WindowStartupLocation="CenterScreen" Height="400" Width="300">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>

        <Grid.Resources>
            <DataTemplate DataType="{x:Type local:PersonModel}">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition/>
                        <ColumnDefinition Width="Auto"/>
                    </Grid.ColumnDefinitions>
                    <TextBlock Text="{Binding Name}" Margin="10 3"/>
                    <TextBlock Text="{Binding Age}" Margin="10 3"
                               Grid.Column="1"/>
                </Grid>
            </DataTemplate>
        </Grid.Resources>

        <ListBox ItemsSource="{Binding Persons}">
            <ListBox.ItemContainerStyle>
                <Style TargetType="ListBoxItem">
                    <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
                </Style>
            </ListBox.ItemContainerStyle>
        </ListBox>
        <Button Content="Randomize" Padding="10 5" Margin="10"
                HorizontalAlignment="Center" VerticalAlignment="Center"
                Grid.Row="1" Click="Button_Click"/>
    </Grid>

</Window>

public abstract class ObservableBase : INotifyPropertyChanged
{
    public void Set<TValue>(ref TValue field, TValue newValue, [CallerMemberName] string propertyName = "")
    {
        if (EqualityComparer<TValue>.Default.Equals(field, default(TValue)) || !field.Equals(newValue))
        {
            field = newValue;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

public class PersonModel : ObservableBase
{
    private string name;
    public string Name
    {
        get { return name; }
        set { Set(ref name, value); }
    }

    private int age;
    public int Age
    {
        get { return age; }
        set { Set(ref age, value); }
    }
}

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = this;
        Mock();
    }

    public ObservableCollection<PersonModel> Persons { get; set; }
        = new ObservableCollection<PersonModel>();

    private Random rand = new Random();

    private void Mock()
    {
        for (int i = 0; i < 10; i++)
        {
            Persons.Add(new PersonModel
            {
                Name = string.Format("Person {0}", i),
                Age = rand.Next(20, 50)
            });
        }
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        foreach (var person in Persons)
        {
            person.Age = rand.Next(20, 50);
        }
    }
}