XAML: Curved Corner Rectangle *Update

Simple. <Rectangle /> cannot have curved corner.* If you want a curved corner filled rectangle, use <Border />.

Example:

<Border x:Name="ShapeBorder" Height="25" Background="Aqua" CornerRadius="3"
 BorderThickness="1" BorderBrush="Black" />
 

The key attributes is CornerRadius! :)

* Ivan told me that, actually you can! Check out the RadiusX and RadiusY properties!