public void IsValid()
        {
            var attribute = new MustBeNullOrEmptyAttribute();

            Assert.AreEqual(true, attribute.IsValid(null));
            Assert.AreEqual(true, attribute.IsValid(DBNull.Value));
            Assert.AreEqual(true, attribute.IsValid(string.Empty));
            Assert.AreEqual(false, attribute.IsValid("aaa"));
            Assert.AreEqual(true, attribute.IsValid(Array.Empty <string>()));
            Assert.AreEqual(false, attribute.IsValid(new string[] { "aaa" }));
            Assert.AreEqual(true, attribute.IsValid(Array.Empty <int>()));
            Assert.AreEqual(false, attribute.IsValid(new int[] { 1 }));
        }
        public void IsValidFail(object value)
        {
            var attribute = new MustBeNullOrEmptyAttribute();

            try
            {
                attribute.IsValid(value);

                Assert.Fail();
            }
            catch (ArgumentException ex)
            {
                Assert.AreEqual("Value must be subtype of IEnumerable.", ex.Message);
            }
        }