public void InvalidInputTriangleTest()
 {
     try
     {
         Triangle tri = new Triangle(3.5, -4.5);
     }
     catch (ArgumentException e)
     {
         Assert.AreEqual("Size must be a positive number.", e.Message);
     }
 }
        public void ValidInputTriangleTest()
        {
            Triangle tri = new Triangle(3.5, 4.5);

            Assert.AreEqual("7.875", string.Format("{0:f3}", tri.CalculateSurface()));
        }