For metro style app, unable to use attachable property defined by other assembly.

BlankPage.xaml

<!--
<Page
    x:Class="APTest.App.BlankPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:APTest.App"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    xmlns:fw="clr-namespace:AP.Framework;assembly=AP.Framework"
    >
-->
<Page
    x:Class="APTest.App.BlankPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:APTest.App"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    xmlns:fw="using:AP.Framework"
    >

    <Grid Background="{StaticResource ApplicationPageBackgroundBrush}" x:Name="ContentRoot">
        <Button x:Name="btn1" Content="ABC" fw:TestBehaviour.Num="200" />
    </Grid>
</Page>

TestBehaviour.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Windows.UI.Xaml;

namespace AP.Framework {
    public static class TestBehaviour {
        public static readonly DependencyProperty NumProperty = DependencyProperty.RegisterAttached(
            "Num",
            typeof(int),
            typeof(TestBehaviour),
            new PropertyMetadata(100)
        );

        public static int GetNum(UIElement d) {
            return (int)d.GetValue(NumProperty);
        }

        public static void SetNum(UIElement d, int value) {
            d.SetValue(NumProperty, value);
        }
    }
}

modified declalation of external assembly namespace