WPF DataGrid not updating on PropertyChanged

i've a problem updating my datagrid when clicking the button by using NotifyPropertyChanged. It works if i set the DataGrid.ItemsSource in code behind, but it doesn't if i set it in xaml. here's some code of code behind & xaml:

namespace MyWpfDataBindingLab
{
public partial class NpcWindow : Window
{
    DataCollection dc = new DataCollection();

    public NpcWindow()
    {
        InitializeComponent();
        //command binding code
        //...
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        //if i set the ItemsSource here, updating of the UI works
        //dataGrid1.ItemsSource = dc;
    }

    private void CmdCollectionChangedExecute(object sender, ExecutedRoutedEventArgs e)
    {
        foreach (SampleClass s in dc)
        {
            s.Property1 = "changed";
            s.Property3 = "changed";
            s.Property3 = "changed";
            break;
        }

        dc.Add(new SampleClass("new sample 1", "new sample 2", "new sample 3"));
    }
}
}

<Window x:Class="WPFDataBinding.NpcWindow"
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:npc="clr-namespace:WPFDataBinding.NotifyPropChanged"
    Title="MainWindow" Height="189" Width="459" Loaded="Window_Loaded">
<Window.Resources>
    <npc:DataCollection x:Key="dataCol"/>
</Window.Resources>
<Grid>
    <Grid.ColumnDefinitions>
    </Grid.ColumnDefinitions>
    <Button Content="Button" Height="23" HorizontalAlignment="Left" Margin="349,110,0,0" Name="button1" VerticalAlignment="Top" Width="75" />
    <!-- if i set the ItemsSource here, updating of the UI doesn't work -->
    <DataGrid ItemsSource="{Binding Source={StaticResource dataCol}, Mode=OneWay, NotifyOnSourceUpdated=True, UpdateSourceTrigger=PropertyChanged}"
              AutoGenerateColumns="True" Height="103" HorizontalAlignment="Left" M