public void TestImmutableKeys()
        {
            AVUser user = new AVUser();
            string[] immutableKeys = new string[] { "sessionToken", "isNew" };

            foreach (var key in immutableKeys)
            {
                Assert.Throws<InvalidOperationException>(() =>
                  user[key] = "1234567890"
                );

                Assert.Throws<InvalidOperationException>(() =>
                  user.Add(key, "1234567890")
                );

                Assert.Throws<InvalidOperationException>(() =>
                  user.AddRangeUniqueToList(key, new string[] { "1234567890" })
                );

                Assert.Throws<InvalidOperationException>(() =>
                  user.Remove(key)
                );

                Assert.Throws<InvalidOperationException>(() =>
                  user.RemoveAllFromList(key, new string[] { "1234567890" })
                );
            }

            // Other special keys should be good
            user["username"] = "******";
            user["password"] = "******";
        }