Reference type instances for defaults in Dependency Property

// Example of what happens when you use a reference type (collections, etc)
// as the default value of a dependency property.
// Not good! Don't do it!
void Main()
{
    var bad1 = new BadIdea();
    var bad2 = new BadIdea();
    bad1.Items.Add(1);
    bad1.Items.Add(2);
    bad2.Items.Dump("Bad Instance 2 has data in it's collection, even though I added it via instance 1.");

    var better1 = new BetterIdea();
    var better2 = new BetterIdea();
    better1.Items.Add(1);
    better1.Items.Add(2);
    better2.Items.Dump("Better Instance 2 works as expected. No Items in collection.");
}

// Don't do it this way...
class BadIdea : DependencyObject
{
    public ObservableCollection<int> Items
    {
        get { return (ObservableCollection<int>)GetValue(ItemsProperty); }
        set { SetValue(ItemsProperty, value); }
    }

    public static readonly DependencyProperty ItemsProperty =
        DependencyProperty.Register(
            "Items",
            typeof(ObservableCollection<int>),
            typeof(BadIdea),
            new UIPropertyMetadata(new ObservableCollection<int>())
        );
}

// Do it this way... with the constructor
class BetterIdea : DependencyObject
{

    public BetterIdea()
    {
        Items = new ObservableCollection<int>();
    }

    public ObservableCollection<int> Items
    {
        get { return (ObservableCollection<int>)GetValue(ItemsProperty); }
        set { SetValue(ItemsProperty, value); }
    }

    public static readonly DependencyProperty ItemsProperty =
        DependencyProperty.Register(
            "Items",
            typeof(ObservableCollection<int>),
            typeof(BetterIdea),
            new UIPropertyMetadata(null)
        );
}