<Window x:Class="MySimpleProgram.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="725"
        >
    <StackPanel Name="StackPanel1" Orientation="Horizontal">

        <TextBox Name="TextBox2" Text="{Binding Path=FirstName, Mode=TwoWay}" Height="23"/>

        <Button Name="Button1" Content="Change C# obj people[0]" Width="175" Height="20" Click="Button1_Click" />

        <ListBox Name="listPeople" DisplayMemberPath="FirstName"/>

    </StackPanel>
</Window>

public class Person : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private String _FirstName;

    public string FirstName
    {
        get { return _FirstName; }
        set
        {
            _FirstName = value;
            if (PropertyChanged != null)
                PropertyChanged(
                    this, new PropertyChangedEventArgs("FirstName"));
        }
    }

    public int Age { get; set; }
}

public partial class MainWindow : Window
{
    public Person[] people;

    public MainWindow()
    {
        InitializeComponent();

        people = new Person[]{
            new Person{ FirstName = "Shirley", Age = 22 },
            new Person{ FirstName = "Roy", Age = 29 },
            new Person{ FirstName = "Manuel", Age = 34 } };

        StackPanel1.DataContext = people[0];

        listPeople.ItemsSource = people;
    }


    private void Button1_Click(object sender, RoutedEventArgs e)
    {
        people[0].FirstName += "y";
    }

}