WPF: Stretch rectangle if aspect ratio > 16:9, scale otherwise

Question

I'm trying to put a rectangle in a grid in a window that will change size regularly. I'm not working with absolute values, but with ratios.

So, there are three states the rectangle could have relative to the window/grid:

• The default aspect ratio for the window is 16:9. If the window has that size, the rectangle should fit into the window perfectly, filling the window;
• If the window's width is bigger than that, the rectangle should stretch with it. (So if the window's aspect ratio > 16/9, the rectangle stretches its width, thus still filling the entire window);
• If the window's height is bigger than the 16:9 ratio, the rectangle inside should (1) not stretch vertically, and (2) align to the bottom of the grid.

This image explains it a lot clearer

I'm looking for a solution that doesn't involve changing code other than XAML, (so nothing in the .cs file), unless there is no other way. I did try finding a solution with C# code though:

RectName_OnSizeChanged(object sender, SizeChangedEventArgs) {
    RectName.MaxHeight = 9/16 * RectName.Width;
}

but it doesn't seem to be working. (So why that is, is my bonus question)

Answer 1

How about this:

<Grid Background="CornflowerBlue" SizeChanged="ParentSizeChanged">
    <Rectangle x:Name="theRect" Fill="Blue" HorizontalAlignment="Left" VerticalAlignment="Bottom" />
</Grid>

And this:

private void ParentSizeChanged(object sender, SizeChangedEventArgs e)
{
    var parent = sender as FrameworkElement;
    if (parent == null)
        return;
    theRect.Width = parent.ActualWidth;
    theRect.Height = Math.Min(parent.ActualHeight, parent.ActualWidth * 6 / 9);
}