Abstraction.Rectangle.GetSurface C# (CSharp) Method

        public override double GetSurface()
        {
            if (this.Width == null)
            {
                throw new InvalidOperationException("Rectangle's width is not provided!");
            }

            if (this.Height == null)
            {
                throw new InvalidOperationException("Rectangle's height is not provided!");
            }

            double surface = this.Width.Value * this.Height.Value;
            return surface;
        }
    }

 public static void Main()
 {
     Circle circle = new Circle(5);
     Console.WriteLine("I am a circle. " +
         "My perimeter is {0:f2}. My surface is {1:f2}.",
         circle.GetPerimeter(), circle.GetSurface());
     Rectangle rect = new Rectangle(2, 3);
     Console.WriteLine("I am a rectangle. " +
         "My perimeter is {0:f2}. My surface is {1:f2}.",
         rect.GetPerimeter(), rect.GetSurface());
 }