ClassHierarchy.cs

namespace Generics;

public class Shape
{
    public required Rect BoundingBox { get; set; }
}

public class RoundedRectangle : Shape
{
    public required double CornerRadius { get; set; }
}

public class BoxAreaComparer : IComparer<Shape>
{
    public int Compare(Shape? x, Shape? y)
    {
        if (x is null)
        {
            return y is null ? 0 : -1;
        }
        if (y is null)
        {
            return 1;
        }

        double xArea = x.BoundingBox.Width * x.BoundingBox.Height;
        double yArea = y.BoundingBox.Width * y.BoundingBox.Height;

        return Math.Sign(xArea - yArea);
    }
}

public class CornerSharpnessComparer : IComparer<RoundedRectangle>
{
    public int Compare(RoundedRectangle? x, RoundedRectangle? y)
    {
        if (x is null)
        {
            return y is null ? 0 : -1;
        }
        if (y is null)
        {
            return 1;
        }

        // Smaller corners are sharper, so smaller radius is "greater" for
        // the purpose of this comparison, hence the backward subtraction.
        return Math.Sign(y.CornerRadius - x.CornerRadius);
    }
}