A rectangle with curved corners

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="WpfApplication1" Height="250" Width="420" WindowStartupLocation="CenterScreen">
    <StackPanel>
    <Rectangle RadiusX ="25" RadiusY ="80" Fill ="DarkBlue" Width ="180" Height ="50"/>
    </StackPanel>
</Window>