public void CacheDictionary_Enumerate()
        {
            var cd = new CacheDictionary<string, int>(EqualityComparer<string>.Default);

            cd.GetOrAdd("foo", s => s == null ? 0 : s.Length);
            cd.GetOrAdd(key: null, s => s == null ? 0 : s.Length);

            var res = cd.ToArray();

            Assert.AreEqual(2, res.Length);

            Assert.IsTrue(res.Any(x => x.Key == null && x.Value == 0));
            Assert.IsTrue(res.Any(x => x.Key == "foo" && x.Value == 3));

            Assert.IsNotNull(((IEnumerable)cd).GetEnumerator());
        }