public void FilterTest()
        {
            // Given
            var a = new[] { null, "", "David", "Hong" };

            // When we collect non-null/empty strings
            var b = a.Filter(x => !string.IsNullOrEmpty(x));

            // Then
            Assert.AreEqual(4, a.Length);
            Assert.AreEqual(2, b.Length);
            Assert.AreEqual("David", b[0]);
        }