Making a Viewbox scale vertically but stretch horizontally

Question

I want to make a Viewbox (or something similar) that scales only its height, and then stretches its content horizontally.

If I do this:

Answer 1

I'm pretty sure the answer is "not easily".

Here's an idea that seems to work but it's kind of clunky:

1. Throw your StackPanel into a UserControl (UserControl1 in the example below).

2. Place two copies of this UserControl into a container (a Grid in the example below).

   a. Align the first copy top/left so that it remains at its default size. This copy is strictly for measurement purposes and should have its Visibility set to Hidden.

   b. Place the second copy inside a Viewbox. This copy is the one the user will actually see.

3. Use a MultiBinding with a MultiValueConverter to adjust the width of the second UserControl so that it has the correct aspect ratio before it gets expanded by the Viewbox.

Here's the markup:

Here's the MultiValueConverter

public class WidthAdjuster : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        var rawHeight = (double)values[0];
        var containerWidth = (double)values[1];
        var containerHeight = (double)values[2];
        var ratio = containerWidth / containerHeight;
        return rawHeight * ratio;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Result for 525 x 350 container