How to draw a rectangle on a WPF canvas

I'm trying to draw a Rectangle on a Canvas as follows:

System.Windows.Shapes.Rectangle rect;
rect = new System.Windows.Shapes.Rectangle();
rect.Stroke = new SolidColorBrush(Colors.Black);
rect.Fill = new SolidColorBrush(Colors.Black);
rect.Width=200;
rect.Height=200;
Canvas.SetLeft(rect,0);
Canvas.SetTop(rect,0);
front_canvas.Children.Add(rect);

Why would this code not draw a rectangle?

The canvas is defined in the associated XAML as follows:

<Canvas Height="200" Width="200" Name="front_canvas" Grid.Row="1" Grid.Column="0">      
</Canvas>

The canvas shows up fine. I can tell because of the gap it leaves in the layout grid.