Rectangles and Grids

[lunchboxtheman]

Hey all,
I'm running into a weird issue with trying to get a rectangle to span an entire list item. Check out the screenshot attachment.

For the first list item there, I used the following XAML

<ListBoxItem HorizontalContentAlignment="Stretch">
. . .
<Rectangle Fill="Red" Grid.ColumnSpan="2" Grid.RowSpan="3" />
. . .

For the second, I'm using the C# code below

            Rectangle highlightRect = new Rectangle();
            Grid.SetColumnSpan(highlightRect, 2);
            Grid.SetRowSpan(highlightRect, 3);
            SolidColorBrush myBrush = new SolidColorBrush(Colors.Red);
            highlightRect.Fill = myBrush;

            . . .

            layoutGrid.Children.Add(highlightRect);

            . . .

            ListBoxItem theItem = new ListBoxItem();
            theItem.HorizontalContentAlignment = HorizontalAlignment.Stretch;
            theItem.Content = layoutGrid;

The first result, the one obtained from using that XAML, is the desired one. What am I not doing in the C# to make the rectangle fill up the rest of the space?

Thanks,
Lunchboxtheman