Set dependency properties in XAML when the base class is shared

Question

As in the title itself, how can you set the dependency property in XAML when the base class is shared? When I try to do this, I get a NullReferenceException, setting the property from code that works fine. It also works when the base class is not shared. I am using .NET4

Here is a sample code to demonstrate:

Windowbase.cs

using System.Windows;

namespace GenericDependencyPropertyTest
{
    public class WindowBase<ViewModel> : Window
    {
        public static readonly DependencyProperty HeaderProperty = DependencyProperty.Register(
        "Header", typeof(string), typeof(WindowBase<ViewModel>), new PropertyMetadata("No Header Name Assigned"));

        public string Header
        {
            get { return (string)GetValue(HeaderProperty); }
            protected set { SetValue(HeaderProperty, value); }
        }
        protected virtual ViewModel Model { get; set; }
    }
}

MainWindow.xaml

<local:WindowBase x:Class="GenericDependencyPropertyTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:GenericDependencyPropertyTest"
        x:TypeArguments="local:IMyViewModel"
        Title="MainWindow" Height="350" Width="525" Header="Test">
    <Grid>

    </Grid>
</local:WindowBase>

MainWindow.xaml.cs

namespace GenericDependencyPropertyTest
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : WindowBase<IMyViewModel>
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        protected override IMyViewModel Model
        {
            get
            {
                return base.Model;
            }
            set
            {
                base.Model = value;
            }
        }
    }
}

IMyViewModel.cs

namespace GenericDependencyPropertyTest
{
    public interface IMyViewModel
    {
    }
}

Answer 1

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public class WindowBase : Window
{
    public static readonly DependencyProperty HeaderProperty = DependencyProperty.Register(
        "Header", typeof(string), typeof(WindowBase), new PropertyMetadata("No Header Name Assigned"));

    public string Header
    {
        get { return (string)GetValue(HeaderProperty); }
        protected set { SetValue(HeaderProperty, value); }
    }
}

public class WindowBase<ViewModel> : WindowBase
{
    protected ViewModel Model { get; set; }
}