WPF: Can only bind to List if DataContext set directly - wpf

Question

I'm learning WPF. Thanks in advance for the help.
I have an object, Directory, that acts as a container for a List of Person objects. I can't figure out why I can't bind my ListBox to the list of Person unless I set the DataContext directly. In other words, I can't use dot notation to access the list as a sub-property of the directory.
Observe the the last line of the C# sharp code below: I set the DataContext to this.directory.People and it works great.
But if I set the DataContext simply to this (to refer to the whole Window) and then try to use dot notation to set my binding like <ListBox ItemsSource="{Binding Path=directory.People}" /> my ListBox is blank.
XAML listed below. Observe the last line of the XAML.
CodeBehind:
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
}
public class Directory
{
public List<Person> People = new List<Person>();
public Directory()
{
this.People.Add(new Person() { Name = "Joseph", Age = 34 });
this.People.Add(new Person() { Name = "Teresa", Age = 29});
this.People.Add(new Person() { Name = "Kulwant", Age = 66 });
this.People.Add(new Person() { Name = "Hyunh", Age = 61});
this.People.Add(new Person() { Name = "Marcio", Age = 65 });
}
}
public partial class MainWindow : Window
{
public Directory directory { get; } = new Directory();
public MainWindow()
{
InitializeComponent();
this.DataContext = this.directory.People;
}
}
XAML:
<Window x:Class="WtfDataTrigger.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:t="clr-namespace:System.Threading;assembly=mscorlib"
xmlns:local="clr-namespace:LearningWPF"
mc:Ignorable="d"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<DataTemplate DataType="{x:Type local:Person}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="1*" />
<RowDefinition Height="1*" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="50" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Label Name="NameLabel" Margin="2" Content="Name" Grid.Column="0" Grid.Row="0" VerticalAlignment="Center" FontWeight="Bold"/>
<TextBlock Name="NameText" Margin="2" Text="{Binding Path=Name}" Grid.Column="1" Grid.Row="0" VerticalAlignment="Center" />
<Label Name="AgeLabel" Margin="2" Content="Age" Grid.Column="0" Grid.Row="1" VerticalAlignment="Center" FontWeight="Bold" />
<TextBlock Name="AgeText" Margin="2" Text="{Binding Path=Age}" Grid.Column="1" Grid.Row="1" VerticalAlignment="Center"/>
</Grid>
</DataTemplate>
</Window.Resources>
<StackPanel>
<ListBox ItemsSource="{Binding}" />
</StackPanel>
</Window>

Answer 1

WPF data binding works with public properties only. While directory is a public property (but should be named Directory), People is a public field.
Change
public List<Person> People = new List<Person>();
to
public List<Person> People { get; } = new List<Person>();