base class property always null when creating objects from xaml - xaml

Question

I'm trying to instance an object(s) from xaml. The object's class inherits from a base class. Everything works good except that the a base class property ("Key") is not setting correctly from xaml. It's always null. The object's properties itself are set OK from xaml. Also when I set the Key property from code it sets fine.
I put a breakpoint on the closing bracket of the MainWindow method to view the object data. The hover details tells me the Key property is always null.
Any ideas what I'm doing wrong?
<?xml version="1.0" encoding="utf-8" ?>
<GroupUiItem xmlns="clr-namespace:Configurator.UiCore"
Key="key_grp1" UserName="grp1">
<ParameterUiItem Key="key_par1" UserName="par1"/>
<GroupUiItem Key="key_grp2" UserName="grp2">
<ParameterUiItem Key="key_par2" UserName="par2"/>
<ParameterUiItem Key="key_par3" UserName="par3"/>
</GroupUiItem>
<ParameterUiItem Key="key_par4" UserName="par4"/>
<ParameterUiItem Key="key_par5" UserName="par5"/>
<ParameterUiItem Key="key_par6" UserName="par6"/>
</GroupUiItem>
public partial class MainWindow : Window
{
public MainWindow()
{
GroupUiItem ConfigUi = new GroupUiItem();
InitializeComponent();
using (FileStream stream = new FileStream("XMLFile1.xaml", FileMode.Open, FileAccess.Read))
{
ConfigUi = XamlReader.Load(stream) as GroupUiItem;
}
ConfigUi.Key = "key_grp1"; // this works OK
CategoryList.ItemsSource = ConfigUi.Children;
}
}
// These are in the Configurator.UiCore namespace:
public class ConfiguratorUiItem
{
protected string _Key;
public string Key
{
get { return _Key; }
set { _Key = value; }
}
}
[ContentProperty("Children")]
public class GroupUiItem : ConfiguratorUiItem
{
private ObservableCollection<ConfiguratorUiItem> _Children = new ObservableCollection<ConfiguratorUiItem>();
public ObservableCollection<ConfiguratorUiItem> Children
{ get { return _Children; }
set { _Children = value; }
}
private string _UserName;
public string UserName
{ get { return _UserName; }
set { _UserName = value; }
}
}
public class ParameterUiItem : ConfiguratorUiItem
{
private string _ParameterType;
public string ParameterType
{
get { return _ParameterType; }
set { _ParameterType = value; }
}
private string _UserName;
public string UserName
{
get { return _UserName; }
set { _UserName = value; }
}
}

Answer 1

OK figured out my problem. Noob mistake. Needed to set build action to None and copy always. I had build action set to a page so it wasn't a loose xaml and wasn't updating to appropriate folder. I copied the xaml file to output directory manually when I first couldn't figure out the problem. This caused the program to always use the old file.
When I did this also had to add ";assembly=Configurator" to the end of the xmlns so that it now reads: "xmlns="clr-namespace:Configurator.UiCore;assembly=Configurator". Then it worked.