public void InvalidInputRectangleTest()
 {
     try
     {
         Rectangle rect = new Rectangle(-3.5, 4.5);
     }
     catch (ArgumentException e)
     {
         Assert.AreEqual("Size must be a positive number.", e.Message);
     }
 }
        public void ValidInputRectangleTest()
        {
            Rectangle rect = new Rectangle(3.5, 4.5);

            Assert.AreEqual("15.75", string.Format("{0:f2}", rect.CalculateSurface()));
        }