See forum post: https://www.noesisengine.com/forums/viewtopic.php?t=2813

public partial class UserControlA : UserControl
{
public static readonly DependencyProperty ValueProperty =
DependencyProperty.Register("Value", typeof(int), typeof(UserControlA), new PropertyMetadata(5, OnValueChanged));
...
}

public partial class UserControlB : UserControl
{
public static readonly DependencyProperty ValueProperty =
UserControlA.ValueProperty.AddOwner(typeof(UserControlB), new PropertyMetadata(OnValueChanged));
...
}

Instantiating both of these classes in XAML causes runtime exception.

Specifying default value in UserControlB DP registration causes UserControlA.OnValueChanged to be called when UserControlB.Value changes.