WPF GetTemplateChild get Nullrefrence exception

Question

Hi I'm building a control I need to access checkbox in the code behind But I get the null error

this is my control

<Style TargetType="TreeViewItem" x:Key="CheckTreeViewItem" >
        <Setter Property="IsExpanded" Value="{Binding Path=IsExpanded,Mode=TwoWay}"/>
    </Style>

    <Style TargetType="local:CheckTreeView">
        <Setter Property="ItemContainerStyle" Value="{StaticResource CheckTreeViewItem}"/>
        <Setter Property="ItemTemplate">
            <Setter.Value>
                <HierarchicalDataTemplate DataType="{x:Type local:CheckTreeSource}" ItemsSource="{Binding Children}">
                    <CheckBox x:Name="PART_CheckBox" Margin="1" IsChecked="{Binding IsChecked}">
                        <TextBlock Text="{Binding Text}"/>
                    </CheckBox>
                </HierarchicalDataTemplate>
            </Setter.Value>
        </Setter>
    </Style>

And this is how I access control

[TemplatePart(Name = CheckBox_Key, Type = typeof(CheckBox))]

    public partial class CheckTreeView : TreeView
    {
        private const string CheckBox_Key = "PART_CheckBox";
        CheckBox checkBox;

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            checkBox = GetTemplateChild(CheckBox_Key) as CheckBox;
            checkBox.Click += CheckBox_Click;
        }

        private void CheckBox_Click(object sender, RoutedEventArgs e)
        {

        }
    }

When I use the following code I get no null error but no control in runtime

static CheckTreeView()
            {
                DefaultStyleKeyProperty.OverrideMetadata(typeof(CheckTreeView),
                       new FrameworkPropertyMetadata(typeof(CheckTreeView)));
            }

Answer 1

To subscribe to Click event of each CheckBox:

    public partial class CheckTreeView : TreeView
    {
        public CheckTreeView()
        {
            InitializeComponent();
            processNode(this);
        }


        void processNode(ItemsControl node)
        {
            node.ItemContainerGenerator.StatusChanged += (sender, args) =>
            {
                ItemContainerGenerator itemContainerGenerator = ((ItemContainerGenerator)sender);
                if (itemContainerGenerator.Status == GeneratorStatus.ContainersGenerated)
                {
                    for (int i = 0; i < itemContainerGenerator.Items.Count; i++)
                    {
                        TreeViewItem treeViewItem =
                            (TreeViewItem) itemContainerGenerator.ContainerFromIndex(i);

                        treeViewItem.Loaded += (o, eventArgs) =>
                        {
                            CheckBox checkBox = FindVisualChild<CheckBox>(treeViewItem);
                            checkBox.Click += CheckBox_Click;
                        };

                        processNode(treeViewItem);
                    }
                }
            };
        }

        public static T FindVisualChild<T>(DependencyObject obj) where T : DependencyObject
        {
            if (obj != null)
            {
                for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
                {
                    var child = VisualTreeHelper.GetChild(obj, i);
                    if (child is T)
                    {
                        return (T)child;
                    }

                    T childItem = FindVisualChild<T>(child);
                    if (childItem != null) return childItem;
                }
            }
            return null;
        }

        private void CheckBox_Click(object sender, RoutedEventArgs e)
        {

        }
    }

Note that if you use ObservableCollection as items source, you should process changes in that collection: subscribe to events of added items, unsubscribe from events of removed items.