The RadiusX and RadiusY properties of the Rectangle class are used to set the roundness of a rectanble. The following code creaetes a rounded rectangle.
/// <summary>
/// Creates a rounded blue rectangle with black border
/// </summary>
public void CreateARoundedRectangle()
{
// Create a Rectangle
Rectangle blueRectangle = new Rectangle();
blueRectangle.Height = 100;
blueRectangle.Width = 200;
// Create a blue and a black Brush
SolidColorBrush blueBrush = new SolidColorBrush();
blueBrush.Color = Colors.Blue;
SolidColorBrush blackBrush = new SolidColorBrush();
blackBrush.Color = Colors.Black;
// Set Rectangle's width and color
blueRectangle.StrokeThickness = 4;
blueRectangle.Stroke = blackBrush;
// Fill rectangle with blue color
blueRectangle.Fill = blueBrush;
// Set roundness
blueRectangle.RadiusX = 20;
blueRectangle.RadiusY = 20;
// Add Rectangle to the Grid.
LayoutRoot.Children.Add(blueRectangle);
}