public void GetKey()
        {
            Assert.Throws <ArgumentOutOfRangeException>(() => _dictionary.GetKey(-1));
            Assert.Throws <ArgumentOutOfRangeException>(() => _dictionary.GetKey(3));

            Assert.AreEqual("one", _dictionary.GetKey(0));
            Assert.AreEqual("three", _dictionary.GetKey(1));
            Assert.AreEqual("two", _dictionary.GetKey(2));
        }