Binding CheckBoxes in a Hierachical datatemplate doesn't seem to work properly

Question

Can anyone explain me why the children checkboxes are not updated by the change in the parent checkbox in the following example?
And second question: How do I get back the the datacontext on a CheckBox Click event?
Thanks in advance,
Dan

using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;

namespace TestCheckBoxBinding
{
    public class TestsViewModel
    {
        public static TestsViewModel ExampleDataContext
        {
            get
            {
                TestsViewModel vm = new TestsViewModel();
                TestCategory cat1 = new TestCategory() {Name = "First category"};
                Test t1 = new Test() {Name = "Test1"};
                Test t2 = new Test() {Name = "Test2"};
                Test t3 = new Test() {Name = "Test3"};
                t1.Children.Add(t2);
                t1.Children.Add(t3);
                Test t4 = new Test() {Name = "Test4"};
                cat1.Tests.Add(t1);
                cat1.Tests.Add(t4);
                vm.Categories.Add(cat1);
                return vm;
            }
        }

        private readonly IList<TestCategory> categories = new List<TestCategory>();
        public IList<TestCategory> Categories { get { return categories; } }
    }

    public class TestCategory
    {
        public string Name { get; set; }
        private readonly IList<Test> tests = new List<Test>();
        public IList<Test> Tests { get { return tests; } }
    }

    public class Test : INotifyPropertyChanged
    {
        private string name;
        public string Name
        {
            set
            {
                if (name != value)
                {
                    name = value;
                    if (PropertyChanged != null)
                        PropertyChanged(this, new PropertyChangedEventArgs("Name"));
                }
            }
            get { return name; }
        }

        public bool? isChecked = false;
        public bool? IsChecked
        {
            get { return isChecked; }
            set
            {
                if (isChecked != value)
                {
                    isChecked = value;
                    if (children.Count() > 0)
                    {
                        foreach (var test in children)
                        {
                            test.isChecked = value;
                            test.Name += ".";
                        }
                    }
                    if (PropertyChanged != null)
                        PropertyChanged(this,new PropertyChangedEventArgs("IsChecked"));
                }
            }
        }

        private readonly IList<Test> children = new List<Test>();

        public IList<Test> Children
        {
            get { return children; }
        }

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        #endregion
    }

<Window x:Class="TestCheckBoxBinding.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:TestCheckBoxBinding"
    Title="Window1" Height="300" Width="300">
    <Window.Resources>
        <local:TestsViewModel x:Key="dataContext"></local:TestsViewModel>
        <Style TargetType="{x:Type TreeViewItem}">
            <Setter Property="IsExpanded" Value="True"/>
        </Style>
        <HierarchicalDataTemplate DataType="{x:Type local:TestCategory}" ItemsSource="{Binding Tests, Mode=OneTime}">
            <Label Content="{Binding Name}"></Label>
        </HierarchicalDataTemplate>
        <HierarchicalDataTemplate DataType="{x:Type local:Test}" ItemsSource="{Binding Children, Mode=OneTime}">
            <StackPanel Orientation="Horizontal">
                <CheckBox VerticalAlignment="Center"  IsChecked="{Binding IsChecked, Mode=TwoWay}" Focusable="False" Click="CheckBox_Click"></CheckBox>
                <Label Content="{Binding Name}"></Label>
            </StackPanel>
        </HierarchicalDataTemplate>
    </Window.Resources>
    <Grid DataContext="{Binding Source={StaticResource dataContext}, Path=ExampleDataContext}" >
        <TreeView ItemsSource="{Binding Categories, Mode=OneTime}"></TreeView>
    </Grid>
</Window>

Answer 1

Oups,  child checkboxes are not being checked because I wasn't setting their IsChecked property. I was setting the isChecked field, which bypasses the property setter and prevents PropertyChanged from being raised.