public void Add()
        {
            Assert.Throws <ArgumentNullException>(() => _dictionary.Add(null, "a null key"));
            Assert.Throws <ArgumentException>(() => _dictionary.Add("two", "another second value"));
            Assert.Throws <NotSupportedException>((() => _readOnly.Add("bar0", "bar value")));

            for (int i = 0; i < 20; i++)
            {
                string key = String.Format("bar{0}", i);
                _dictionary.Add(key, "bar value");
                Assert.IsTrue(_dictionary.Contains(new StringStringPair(key, "bar value")));
            }
            Assert.AreEqual(23, _dictionary.Count);
        }