WPF Dependency property not firing with binding

Question

I have a class:

public class CommandHamburgerMenu : FrameworkElement, ICommand
{
    public HamburgerMenuItem Item
    {
        get { return (HamburgerMenuItem)GetValue(ItemProperty); }
        set { SetValue(ItemProperty, value); }
    }

    public static readonly DependencyProperty ItemProperty =
        DependencyProperty.Register("Item", typeof(HamburgerMenuItem), typeof(CommandHamburgerMenu), new UIPropertyMetadata(null, new PropertyChangedCallback(ItemChanged)));

    private static void ItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        CommandHamburgerMenu commandHamburgerMenu = (CommandHamburgerMenu)d;
    }

    public event EventHandler CanExecuteChanged;

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        if (this.Item != null)
        {
            if (this.Item == MainWindow.Instance.itemHome) MessageBox.Show("Home item");
            else if (this.Item == MainWindow.Instance.itemSearch) MessageBox.Show("Search item");
            else MessageBox.Show("Other");
        }
        else
        {
            MessageBox.Show("Item is null!");
        }
    }
}

and the XAML code:

<HamburgerMenu:HamburgerMenu MenuIconColor="Black" SelectionIndicatorColor="Black" MenuItemForeground="Black" HorizontalAlignment="Left">
    <HamburgerMenu:HamburgerMenu.Background>
        <SolidColorBrush Color="{Binding Source={x:Static properties:Settings.Default}, Path=ColorHighlight, Mode=TwoWay}" />
    </HamburgerMenu:HamburgerMenu.Background>
    <HamburgerMenu:HamburgerMenuItem x:FieldModifier="public" x:Name="itemHome" Icon="Assets/home.png" Text="Home">
        <HamburgerMenu:HamburgerMenuItem.SelectionCommand>
            <commands:CommandHamburgerMenu Tag="{Binding ElementName=itemHome, Path=Text}" />
        </HamburgerMenu:HamburgerMenuItem.SelectionCommand>
    </HamburgerMenu:HamburgerMenuItem>
    <HamburgerMenu:HamburgerMenuItem x:FieldModifier="public" x:Name="itemSearch" Icon="Assets/search.png" Text="Search">
        <HamburgerMenu:HamburgerMenuItem.SelectionCommand>
            <commands:CommandHamburgerMenu Tag="{Binding ElementName=itemHome, Path=Text}" />
        </HamburgerMenu:HamburgerMenuItem.SelectionCommand>
    </HamburgerMenu:HamburgerMenuItem>
    <HamburgerMenu:HamburgerMenuItem x:FieldModifier="public" x:Name="itemFavorite" Icon="Assets/favorite.png" Text="Likes"/>
    <HamburgerMenu:HamburgerMenuItem x:FieldModifier="public" x:Name="itemList" Icon="Assets/list.png" Text="Lists"/>
    <HamburgerMenu:HamburgerMenuItem x:FieldModifier="public" x:Name="itemPerson" Icon="Assets/person.png" Text="Profile"/>
</HamburgerMenu:HamburgerMenu>

HamburgerMenu is a control taken from here, property SelectionCommand is a dependency property of ICommand type, it's firing by click on hamburger menu item. But when app starts, Item property is null, and ItemChanged not firing. It fires only after manually setting Item property from code behind. Why?

Answer 1

The PropertyChangedCallback of a dependency property is only called when the value of the dependency property is actually changed.

It is not called once initially for the default value of the dependency property.

But you could of course call the method yourself, for example in the constructor of the class:

public class CommandHamburgerMenu : FrameworkElement, ICommand
{
    public CommandHamburgerMenu()
    {
        ItemChanged(this, new DependencyPropertyChangedEventArgs(CommandHamburgerMenu.ItemProperty, null, null));
    }

    public HamburgerMenuItem Item
    {
        get { return (HamburgerMenuItem)GetValue(ItemProperty); }
        set { SetValue(ItemProperty, value); }
    }

    public static readonly DependencyProperty ItemProperty =
        DependencyProperty.Register("Item", typeof(HamburgerMenuItem), typeof(CommandHamburgerMenu), new UIPropertyMetadata(null, new PropertyChangedCallback(ItemChanged)));

    private static void ItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        CommandHamburgerMenu commandHamburgerMenu = (CommandHamburgerMenu)d;
    }

    public event EventHandler CanExecuteChanged;

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        if (this.Item != null)
        {
            if (this.Item == MainWindow.Instance.itemHome) MessageBox.Show("Home item");
            else if (this.Item == MainWindow.Instance.itemSearch) MessageBox.Show("Search item");
            else MessageBox.Show("Other");
        }
        else
        {
            MessageBox.Show("Item is null!");
        }
    }
}