Standard property works, but dependency property doesn't in WPF

Question

I have the following code behind, which works:

public DataTemplate ItemTemplate
{
    get { return _list.ItemTemplate; }
    set { _list.ItemTemplate = value; }
}

And I have the code, that I want to have, but it doesn't work. Even setter is never invoked:

public static readonly DependencyProperty ItemTemplateProperty = DependencyProperty.Register("ItemTemplate", typeof(DataTemplate), typeof(MyUserControl));
public DataTemplate ItemTemplate
{
    get { return (DataTemplate)GetValue(ItemTemplateProperty); }
    set
    {
        _list.ItemTemplate = value;
        SetValue(ItemTemplateProperty, value);
    }
}

The use of that is in XAML:

<Window.Resources>
    <DataTemplate x:Key="ItemTemplate">
        <TextBlock Text="{Binding Path=Name}"/>
    </DataTemplate>
</Window.Resources>

<local:MyUserControl ItemTemplate="{StaticResource ItemTemplate}"/>

Why the standard property works and dependency property doesn't?

Answer 1

With the dependency property, .Net is doing something that's not obvious: it's accessing the dependency property identified by ItemTemplateProperty directly instead of using the get and set methods that you declared. The only difference, in this case, is that your _list.ItemTemplate = value; is never run.

When you use dependency properties, your getters and setters should only contain the usual things. Anything else will end up being confusing because WPF bypasses them when it uses the property.

If you need to set _list.ItemTemplate to the value, you should attach a static PropertyChangedCallback using the other DependencyProperty.Register overload. E.g.

private static void OnItemTemplateChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
    var uc = (MyUserControl)d;
    uc._list.ItemTemplate = (DataTemplate)e.NewValue;
}