vtable.cs

unsafe
{
    Circle circle = new() { Radius = 1 };
    Rectangle rectangle = new() { Width = 2, Height = 3 };
    Rectangle square = new() { Width = 2, Height = 2 };

    Console.WriteLine(Algorithm.AreaRatio(0, &circle));
    Console.WriteLine(Algorithm.AreaRatio(1, &rectangle));
    Console.WriteLine(Algorithm.AreaRatio(1, &square));
}

public struct Circle
{
    public double Radius;
}

struct Rectangle
{
    public double Width;
    public double Height;
}

unsafe class Algorithm
{
    public static double AreaRatio(int vtableIndex, void* shape)
    {
        return RuntimeTimeInfo.Vtables[vtableIndex].Area(shape)
            / RuntimeTimeInfo.Vtables[vtableIndex].Perimeter(shape);
    }
}

unsafe class RuntimeTimeInfo
{
    public struct ShapeVTable
    {
        public string Name;
        public delegate*<void*, double> Area;
        public delegate*<void*, double> Perimeter;
    }

    public static ShapeVTable[] Vtables =
    [
        new()
        {
            Name = "Circle",
            Area = Func<Circle, double>(&Area),
            Perimeter = Func<Circle, double>(&Perimeter),
        },
        new()
        {
            Name = "Rectangle",
            Area = Func<Rectangle, double>(&Area),
            Perimeter = Func<Rectangle, double>(&Perimeter),
        }
    ];

    static delegate*<void*, TResult> Func<TThis, TResult>(delegate*<TThis*, TResult> @delegate) where TThis : unmanaged
    {
        return (delegate*<void*, TResult>)@delegate;
    }

    static double Area(Circle* c)
    {
        return Math.PI * c->Radius * c->Radius;
    }

    static double Perimeter(Circle* c)
    {
        return 2 * Math.PI * c->Radius;
    }

    static double Area(Rectangle* r)
    {
        return r->Width * r->Height;
    }

    static double Perimeter(Rectangle* r)
    {
        return 2 * (r->Width + r->Height);
    }
}