Nonrectangular Window, Irregular Window

<Window x:Class="MyWpfSample.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        AllowsTransparency="True" WindowStyle="None" Background="Transparent"
        Title="Window1" Height="300" Width="300"
        MouseLeftButtonDown="Window_MouseLeftButtonDown">
    <Grid>
        <Ellipse Fill="Red" Opacity="0.5" Margin="20">
            <Ellipse.BitmapEffect>
                <DropShadowBitmapEffect/>
            </Ellipse.BitmapEffect>
        </Ellipse>
        <Button Margin="100" Click="Button_Click">Close</Button>
    </Grid>
</Window>

public partial class Window1 : Window
{
    public Window1()
    {
        InitializeComponent();
    }

    private void Window_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        this.DragMove();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        this.Close();
    }
}

Referenced from book WPF unleashed, page225.