public void TestArrayAdd() {

            var array = new[] { 1, 2, 3 };

            array = array.Add(4);

            Assert.AreEqual(4, array.Length);
            Assert.AreEqual(1, array[0]);
            Assert.AreEqual(2, array[1]);
            Assert.AreEqual(3, array[2]);
            Assert.AreEqual(4, array[3]);
        }